Rolling exurban St. Louis pasture and oak-hickory woodland with a dolomite ledge, chert gravel and cedar in the field edges.
Franklin County County-adopted code, county inspection

Barndominium Builders in Franklin County, MO

Franklin County is where the buildable acreage nearest St. Louis actually is — 922.7 square miles, 1,657 farms on 265,858 acres at a 160-acre average, and 56,227 of the county's 107,256 residents living outside any city limit at all. It is also one of the twenty-five Missouri counties whose statutory classification lets it hold a building code, and it holds one: the Building Department at 400 East Locust in Union permits and inspects new construction throughout unincorporated territory under the 2021 International Residential Code, in effect here since 23 April 2024, with a long set of the county's own amendments attached to it. Two of those amendments decide whether a shop-and-house works on a given parcel before anything else does. Monolithic slabs are allowed only on detached, uninhabitable structures; and all habitable space, plus any uninhabitable space with plumbing, must sit on a continuous frost wall carried to the county's 30-inch frost line. The same document then does something unusual for a county: it prints prescriptive post-frame tables of its own — a post-height capacity table and a pole barn header chart, down to the size of the precast pad under each post — and calls for a Missouri Registered Design Professional's sealed design once a building passes 5,000 square feet or falls outside the chart. Steel-frame construction needs that seal regardless of size. And the county's answer is not the answer inside a city: Washington, Union and Pacific each run their own building department, each adopted its own edition on its own date, and the three of them publish three different design wind speeds inside one county line.

Franklin County at a glance

County
Franklin County
Population
107,256
ZIP codes
63013, 63014, 63015, 63037, 63039, 63041, 63055, 63056, 63060, 63061, 63068, 63069, 63072, 63073, 63077, 63079, 63080, 63084, 63089, 63090, 63091
Site visits
Usually within a few days — Franklin County is the next county west along I-44 and Highway 100
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Building conditions in Franklin County

Missouri sets no residential building code and no statewide way of adopting one, so the first question on any parcel is not what the code says — it is whether there is one. Only counties of the first and second classification may adopt a building code at all, and classification is set by assessed valuation rather than population, so a small county can hold the power while a much larger one does not. Zoning is a separate power with its own answer, granted only after a vote of the people and repealable by another. What changes from one jurisdiction to the next is whether the work is inspected, who issues the permit, what that office charges, how the parcel is zoned, and the design figures your drawings have to satisfy.

Build conditions for Franklin County , Missouri : design wind speed 105 mph in the unincorporated county; 115 mph 3-second gust in Washington and Union; 90 mph 3-second gust in Pacific. BUILD CONDITIONS FRANKLIN COUNTY…Franklin CountyCOUNTY-ADOPTED CODEDESIGN WIND SPEED105 mph in the unin…Three different published figures inside one county, each read from that jurisdiction's ownadopted table. Franklin County's Table R301.2 amendment gives 105 mph. The City of Union's Table…ZONING · W — THE COUNTY'S AGRICULTURAL AND LO…3 acres without central water and se…From Article 7 of the Unified Land Use Regulations of FranklinCounty, adopted 14 January 2020 and revised 1 February 2022 and 14…WHO ISSUES THE PERMITFranklin County Building Department400 E. Locust Street, Room 006, Union, MO 63084, (636) 583-6384 —permits, plan review and inspections for unincorporated territory,…
Build conditions for Franklin County, drawn from the sourced facts on this page. Every figure above is cited in the quick-facts panel below, with its source and vintage.
Monolithic slabs are barred under habitable space
This is the amendment that decides whether a barndominium works here, and almost nobody expects it. Franklin County's amended footing table carries the footnote that monolithic slabs are "allowed only on detached/uninhabitable structures," and its amended R403.1.4.1 adds that "All habitable spaces and any uninhabitable space with plumbing must have a continuous frost wall." The single thickened-edge pour that a great many barndominium packages are drawn around is therefore not a permitted foundation for the living half of the building in unincorporated Franklin County — that half needs footings and a continuous frost wall carried to the county's 30-inch frost line. A detached shop with no plumbing is a different question from the house it stands next to.

Franklin County, "Amendments to the 2021 Residential One- and Two-Family Dwelling Code", amended Table 403.1 footnote (b) and amended R403.1.4.1(1), published by the County Building Department at franklinmo.org (_files/ugd/605d96_b0bb7ab65d7d410e9c3001b2a0466f6e.pdf); downloaded and text-extracted 6 September 2026

The county fills in its own design table — here is the whole row
Franklin County's amendment inserts a completed Table R301.2: ground snow load 20 psf; roof snow load 20 psf; wind speed 105 mph; ice shield underlayment not required; seismic design category C; weathering severe; frost line depth 30 inches; termite moderate to heavy; decay slight to moderate; winter design temperature 0 degrees F; mean annual temperature 56 degrees; air freezing index 1,000; flood hazards by Article 11, Appendix H and the county's flood plain map. The 30-inch frost line is the number that sets the depth of every pier, every post hole and every frost wall on a post-frame building, and it is the figure to hand a foundation quote.

Franklin County, "Amendments to the 2021 Residential One- and Two-Family Dwelling Code", "R301.2 Table. Insert the following"; published by the County Building Department at franklinmo.org (_files/ugd/605d96_b0bb7ab65d7d410e9c3001b2a0466f6e.pdf), downloaded and text-extracted 6 September 2026

Four jurisdictions in one county, three design wind speeds
Franklin County publishes 105 mph. The City of Union publishes 115 mph as a 3-second gust, with 89 mph in a second column its ordinance labels fastest mile. The City of Washington publishes the same 115 mph 3-second gust, with 90 mph labelled fastest mile. The City of Pacific publishes 90 mph as a 3-second gust, with 76 mph labelled fastest mile. All four agree on 20 psf ground snow, a 30-inch frost line and seismic design category C — they part company on wind, and the gap between Pacific and Union is 25 mph on the same convention. Part of that is edition: Pacific is still on the 2018 code while Union and Washington moved to 2021 in late 2023. It does not matter why. What matters is that a design wind speed does not travel across a municipal boundary in this county, and the office that will issue your permit is the only one whose table counts.

Franklin County amended Table R301.2 (as above); City of Union Municipal Code § 500.065 item 8, Table 301.2 (Ord. No. 4636, 13 November 2023); City of Washington Code ch. 500 Attachment 1, Exhibit A, Table R301.2(1) (Ord. No. 23-13877, 4 December 2023, code supplement 8, January 2024); City of Pacific Municipal Code § 500.153 item 12, Table 301.2(1) (Ord. No. 3349, 20 December 2022) — all three municipal codes read on eCode360, 6 September 2026

The adopted residential code is the 2021 IRC, since 23 April 2024
The Franklin County Building Department lists the codes in effect as of 23 April 2024: the 2021 International Building Code, the 2021 International Residential Code, the 2021 International Mechanical Code, the 2021 International Plumbing Code, the 2021 International Fire Code, the 2021 ICC Electrical Code, the 2020 National Electrical Code and the 2021 International Swimming Pool and Spa Code. The residential adoption takes in Appendix Chapters E and J, and two copies of every adopted document are on file with the County Clerk at the Government Center in Union. Worth knowing because the figure most often quoted for this county is out of date: the State of Missouri's open-data table of county building codes still shows Franklin County on the 2015 IRC, and that table has not been refreshed since 2021.

Franklin County Building Department, "Building Codes" listing at https://www.franklinmo.org/building-dept, read 6 September 2026; adoption clause from the County's "Amendments to the 2021 Residential One- and Two-Family Dwelling Code" § 4; Missouri Department of Economic Development, "County Building Codes for Missouri", data.mo.gov dataset iq7s-izvt, last updated 11 August 2021

The county's own code carries pole barn tables
Franklin County wrote post-frame construction into its residential code rather than leaving it to a supplier's drawing. The amendments add Table R602.7(1), a post-height capacity table running from a 4x6 at 8 feet through 6x6 and utility poles to 3-, 4- and 5-ply built-up columns at heights to 22 feet, and Table R602.7(1)(1), a pole barn header chart for post spacings of 8 to 16 feet and clear spans of 20 to 40 feet, at 20 lb live load, 5 lb dead load, no ceiling load, purlin and metal roofing. The footnotes are as specific as the tables: a utility pole must be at least 6 inches on the small end and structurally sound; every header must bear on a post or on a bearing block at least 20 inches long and be bolted or lagged to the post; the bottom figure in each cell is the concrete footing under the post, with precast at 4 by 14 inches; roof purlins are sized to 2021 IRC Table R802.4.1(1); and a Registered Design Professional's design is required for buildings over 5,000 square feet or anything outside the chart.

Franklin County, "Amendments to the 2021 Residential One- and Two-Family Dwelling Code", Tables R602.7(1) and R602.7(1)(1) with footnotes A–E; published by the County Building Department at franklinmo.org (_files/ugd/605d96_b0bb7ab65d7d410e9c3001b2a0466f6e.pdf), downloaded and text-extracted 6 September 2026

There is an agricultural exemption from the county building permit — and it excludes a dwelling in terms
Franklin County is one of the counties that wrote a farm-building exemption into its own building code, and it is worth reading closely because it does the opposite of what buyers hope. The county's amended R105.2 exempts from permit "An Agricultural Building meeting the requirements of the 'Agricultural Structure Exemption'" — and the definition the county attaches, carried forward from Commission Order 02-025 adopted 30 October 2003, is: "A structure designed and constructed to house farm implements, hay, grain, poultry, livestock, or other horticultural products EXCLUSIVELY. This structure shall not be a place of human habitation or a place of employment where agricultural products are processed, treated, or packaged, nor shall it be a place used by the public." Claiming it means a notarised affidavit, a plot plan, an on-site inspection by the Building Department to judge whether agricultural use is feasible, a minimum of 200 linear feet from any residence on that property or a neighbouring one, and continuing consent to inspection if a complaint is filed. If the building is then not used exclusively for agriculture, the county requires a sealed as-built plan from a Registered Design Professional and a doubled permit fee, and reserves injunction and prosecution. The exemption form still routes through Planning and Zoning for setbacks and district approval, so it is never an exemption from zoning either.

Franklin County, "Amendments to the 2021 Residential One- and Two-Family Dwelling Code", amended R105.2 Building item 12 and the attached "Agricultural Structure Exemption" form and plot plan; published by the County Building Department at franklinmo.org (_files/ugd/605d96_b0bb7ab65d7d410e9c3001b2a0466f6e.pdf), downloaded and text-extracted 6 September 2026

Also worth knowing before you build in Franklin County

A soil evaluation or perc test is required regardless of acreage
Missouri's own statute, RSMo 701.031, excludes a single-family residence lot of three acres or more from the state on-site sewage programme. Franklin County does not follow it. The county has run its own On-Site Sewage Disposal Systems Ordinance and Regulations since 1 January 1998 — the current version was adopted and amended on 23 September 2025 and took effect 1 October 2025 — and its permit guide states twice, in identical words, that soil evaluation results or percolation test results, with a detailed site plan, complete sewer design data and a choice of installer, "are mandatory at the time of application — regardless of acreage size." The person who performs the evaluation and the person who installs the system must both be certified and registered with the Franklin County Building Department, not merely with the state. A soil evaluation that fails to meet acceptable permeability rates means an engineered sewer design or possibly a lagoon. Homeowners may install their own system, but must sign an affidavit and stake the entire system for a walk-through with an inspector before the permit is approved.

Franklin County Building Department, "Building in Franklin County — Your Guide to the Permit Process", Informational Booklet 04/2022, items 3a and 3b and the On-Site Septic System Information section, published at franklinmo.org (_files/ugd/395081_59f7e3004ba74fbdbd7bc4664d14a6e4.pdf), downloaded and text-extracted 6 September 2026; current ordinance date from the Building Department page at https://www.franklinmo.org/building-dept, read 6 September 2026; RSMo 701.031, rendered 6 September 2026

The most extensive soil in the county has a fragipan and a perched water table
The single most widespread major soil component in Franklin County is the Union series, at about 82,437 acres of the county's mapped major components — more than any other, and ahead of Useful, Hobson, Wrengart and Goss. The federal series description is blunt about what it is: very deep, moderately well drained soils "formed in a layer of loess that contains a fragipan and in the underlying clayey residuum weathered from cherty limestone or cherty dolomite," with depth to the fragipan of 18 to 36 inches, permeability "moderate above the fragipan and slow in the fragipan," low saturated hydraulic conductivity, and "a zone of saturation perched immediately above the fragipan." Two design consequences follow without any guesswork: water that sits on a dense layer within a foot and a half to three feet of the surface, and a drainfield problem before there is ever a house problem. Rock outcrop is a major component on another 11,281 acres, which is the other half of the Ozark-border picture.

USDA-NRCS Soil Data Access, tabular query of legend/mapunit/component for survey area MO071 (Franklin County), major components ranked by map-unit acres, run 6 September 2026; USDA-NRCS Official Series Description, UNION series (soilseries.sc.egov.usda.gov/OSD_Docs/U/UNION.html), fetched 6 September 2026

37 documented sinkholes, not the Ozark core
Franklin County sits on the northern edge of the Ozarks, and the karst comes with it — but at a scale worth stating plainly rather than implying. A spatial query of Missouri DNR's live sinkhole inventory against the county boundary returns 37 documented sinkhole points inside Franklin County, out of 16,001 statewide. For scale, the same query returns 1,471 in Greene County and 0 in Atchison County. So the honest answer for a Franklin County parcel is that documented sinkholes are real, mapped and uncommon here — which is a reason to pull the parcel on the state's GeoSTRAT viewer before buying rather than a reason to assume the worst. Missouri DNR's own caution applies either way: many sinkholes "are not readily visible on the surface because voids are plugged or capped with soil or thin layers of rock," and may only be found during excavation or drilling.

Missouri DNR / Missouri Geological Survey sinkholes map service, layer 1 SINKHOLES (gis.dnr.mo.gov/host/rest/services/geology/sinkholes/MapServer/1), count-only spatial query against the U.S. Census TIGERweb boundary polygon for county FIPS 29071, run 6 September 2026; statewide, Greene and Atchison counts from the same service on the same day; quotation from Missouri DNR, "Sinkholes" (dnr.mo.gov/land-geology/hazards/sinkholes)

More than half the county lives where the county's rules govern
Thirteen incorporated places sit wholly or partly inside Franklin County — Washington 15,521, Union 13,198, the Franklin County portion of Pacific 6,770, the Franklin County portion of Sullivan 5,423, St. Clair 4,748, New Haven 2,399, Gerald 1,359, Parkway 642, Oak Grove Village 405, Berger 269, Leslie 156, Miramiguoa Park 123 and Charmwood 16. Add them up and 51,029 of the county's 107,256 residents live inside a city; the other 56,227 live in the unincorporated balance, which is the territory the county's building code and the Unified Land Use Regulations actually reach. That majority is the reason this county's own rules are worth reading rather than skipping to a city's.

U.S. Census Bureau Vintage 2024 Subcounty Population Estimates, file sub-est2024_29.csv, SUMLEV 157 and 050 records for county FIPS 29071 (including the "Balance of Franklin County" remainder record), fetched from www2.census.gov 6 September 2026

Zoning here can be voted out, and the rules would go with it
Franklin County zones under the enabling act for first-classification counties without a charter, RSMo 64.211 to 64.295. Two features of that act are worth a landowner's attention. First, it took a vote to create — RSMo 64.211 lets the commission adopt a county plan only "after approval by vote of the people of that county," on the ballot question "Shall county planning be established?" Second, it can be undone the same way: on a petition signed by voters equal to five percent of the county's vote for governor at the last preceding gubernatorial election, RSMo 64.212 puts termination to the electorate, and if a majority votes to terminate, "Any resolution, ordinance or regulation adopted under the program pursuant to the provisions of sections 64.211 to 64.295 shall be void and of no effect." Not merely unenforced — void. The building code is a separate power under separate sections and would not fall with it.

RSMo 64.211 (effective 28 August 1994) and RSMo 64.212 (L. 1979 H.B. 663, A.L. 1994 H.B. 1175, effective 28 August 1994), both rendered from revisor.mo.gov 6 September 2026 against a control test in which a bogus section redirected to /Main/nofish.aspx

The addressing authority, the driveway permit and the floodplain desk
Three gates that are easy to miss and each sit with a different office. Addressing: a County Commission order of 27 August 1998 adopted a road-naming policy and created the Franklin County Addressing Authority for unincorporated areas, and since 2018 the Planning and Zoning Department has been that authority — a street address is requested from it by application. Driveways: an entrance permit is required to build or modify a driveway or entrance onto a county road, the fee is $60.00, any work in the roadway right-of-way carries the same $60.00 charge, and the homeowner buys and installs the culvert. Floodplain: development in the floodplain may require review by the Franklin County Floodplain Administrator and an outside contracted engineer under the county's Flood Damage Prevention Ordinance, on the state's floodplain development permit application, and the county warns that those reviews run longer.

Franklin County Addressing page (https://www.franklinmo.org/addressing), Highway Department driveway page (https://www.franklinmo.org/highway-driveway) and Planning and Zoning Department page (https://www.franklinmo.org/planning-zoning), all read 6 September 2026

1,657 farms, and the land base is holding
The 2022 Census of Agriculture counted 1,657 farms in Franklin County on 265,858 acres, an average holding of 160 acres. Read the changes against each other: the number of farms fell 9 percent since 2017 while land in farms did not move at all and average farm size rose 10 percent — consolidation, not conversion, which is the opposite of what the inner counties show. Of that land, 131,988 acres are cropland, 57,309 pastureland, 63,837 woodland and 12,724 other. By size, 159 farms are 1 to 9 acres, 558 are 10 to 49, 594 are 50 to 179, 254 are 180 to 499, 45 are 500 to 999 and 47 are 1,000 acres or more. That distribution — nearly 1,100 holdings under 180 acres — is the shape of a market where parcels in the range a barndominium buyer wants still change hands.

USDA NASS 2022 Census of Agriculture, County Profile — Franklin County, Missouri (cp29071.pdf), downloaded from www.nass.usda.gov and text-extracted 6 September 2026, against a control in which a bogus county profile returned a genuine 196-byte HTTP 404; county land area 922.684 sq mi from the 2024 Census Gazetteer counties file, fetched 6 September 2026

An excavator grading a compacted building pad on a cleared lot, with survey stakes and a drainage swale visible.
Below the slab

The expensive surprises are in the ground, not the building

What moves a Missouri budget is usually below grade, and which problem you have depends on where in the state you are. Across the Ozarks it is karst: the Department of Natural Resources maps more than sixteen thousand sinkhole points, and what matters there is what is under the slab rather than what the slab sits in. Across the north and centre it is claypan — dense, slow-draining subsoil that perches water above it for months of the year. In the Bootheel it is deep alluvium carrying the highest seismic demand in the state, where the same soils modelled for liquefaction attack post embedment and slab bearing rather than the frame above them. The same ground then decides the septic system, and Missouri does not run that on a percolation test alone: the rule allows a perc test or a soil morphology examination in a profile pit, and the authority reviewing your parcel picks which. Where a drainfield sits near a sinkhole's drainage, perc testing on its own is not permitted at all. Whether that authority is the state or your own county is itself a local question, and it changes what you have to file. So the site work, the foundation and the drainfield get resolved together rather than one at a time. We price the dirt before we price the building.

What we build in Franklin County

All nine services are available in Franklin County. What a build costs depends far more on the parcel than on square footage, so we price against your land rather than publishing a rate card.

Custom Plans

Barndominium floor plans drawn to your parcel and your soil, and to whichever Missouri authority — a county, a city, or none at all — will review them.

Barndominium Kits

Barndominium shell packages delivered to your Missouri land, drawn to the criteria your own county or city publishes — because the state publishes none.

Turnkey Builds

Start to finish barndominium construction in Missouri: the permits that genuinely apply, site work, shell and interior finish, under one contract.

See all nine services

What a build looks like

Three stages every Franklin County project passes through, from a raw parcel to a finished home. Site work first, because that is where the budget is decided.

Aerial view of an undeveloped rural lot with cleared grass, a oak and hickory treeline with a few loblolly pines, and a pale gravel access track.
Stage one

The parcel

Zoning, setbacks, flood zone and utility access get checked before anything is drawn.

An excavator grading a compacted building pad on a cleared lot, with survey stakes and a drainage swale visible.
Stage two

Pad and permit

Clearing, grading and a compacted pad, while the submittal works through whichever office issues the permit where you are building.

A finished barndominium at dusk with warm interior lighting glowing through its large windows.
Stage three

Frame to finish

Shell erected and dried in, then systems, insulation and interior finish through to CO.

Those are the rules where you are building in Franklin County. Tell us what you want to build and we will draw it to them.

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Where we build around Franklin County

Franklin County and the rural land around it. The buildable acreage for a project this size usually sits outside the city limits, which is frequently what changes who issues your permit.

The Franklin County service area, including the rural acreage outside the built-up edge, which is where the parcels big enough for a build like this usually sit. The map marks the territory we build in. It is not a branch location, and there is no office at this pin.

Keep reading

The questions that usually come next once you know what Franklin County allows.

Barndominium Cost Guide

Sourced per-square-foot ranges by tier and by size, with a calculator built on the same figures.

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Is It Legal in Missouri?

Classification, zoning, the HOA gate, and why "nonresidential farm building" does not cover a home.

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How Long Does It Take?

Census build-duration data, and what actually sets the permit clock where you are building.

Read it

Nearby areas we build in

Others we cover nearby, and they are not interchangeable. One may be a county with an adopted building code, another a county with no legal authority to have one, another a city that permits inside its own limits whichever county the land sits in. So the office that issues the permit, whether anyone inspects the work at all, the fees, the zoning and the design figures can differ from one card to the next — sometimes across a single property line.

St. Louis County

St. Louis County
Jefferson County

Jefferson County
Warren County

Warren County
Crawford County

Crawford County

Building in Franklin County: common questions

9 questions we get asked most often about building in Franklin County. If yours is not on the list, ask it directly.

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Can I pour one slab for the whole barndominium in Franklin County?
Not for the part you live in, if the land is in unincorporated Franklin County. The county's amended footing table says monolithic slabs are "allowed only on detached/uninhabitable structures," and its amended R403.1.4.1 adds that all habitable space — and any uninhabitable space with plumbing — must have a continuous frost wall. The frost line here is 30 inches, so the living half of the building needs footings and a frost wall carried to that depth, and a monolithic pour is not an approved substitute. Where a monolithic slab is allowed, the county still sets its minimum reinforcement: two #4 horizontal bars, #4 vertical at 2 feet on centre bent into the slab, a 2-foot by 2-foot grid in the slab, and slab and footing poured at the same time. This is the single most common surprise on a Franklin County barndominium budget, and it is much cheaper to find out before the plans are drawn than after.
Does calling it an agricultural building get me out of the county building permit?
There is a real exemption here, and it will not do what you want. Franklin County's amended R105.2 exempts an agricultural building that meets the county's Agricultural Structure Exemption, and the county's own definition — carried from Commission Order 02-025, adopted 30 October 2003 — describes a structure built to house farm implements, hay, grain, poultry, livestock or other horticultural products exclusively, and says in the same sentence that it "shall not be a place of human habitation." A barndominium is a place of human habitation; that is the whole product. Claiming the exemption also means a notarised affidavit, a plot plan, an on-site inspection to judge whether agricultural use is feasible, at least 200 linear feet of separation from any residence on that property or a neighbouring one, and standing consent to a follow-up inspection if a complaint is filed. If the building later turns out not to be exclusively agricultural, the county requires a sealed as-built plan from a Registered Design Professional and doubles the permit fee, and reserves the right to seek an injunction or prosecution. And the exemption form itself is routed through Planning and Zoning for the district and the setbacks, so it never exempts anything from zoning.
My land is over three acres — do I still need a septic permit and a soil test?
Yes, in Franklin County. The state statute people quote, RSMo 701.031, does exclude a single-family residence lot of three acres or more from the state on-site sewage programme — but it excludes it only from the state programme, and a county ordinance can still reach the same parcel. Franklin County's does. The county has administered its own On-Site Sewage Disposal Systems Ordinance and Regulations since 1 January 1998, the current version took effect 1 October 2025, and the county's permit guide states in terms that soil evaluation or percolation test results with a detailed site plan, complete sewer design data and a choice of installer are mandatory at the time of application "regardless of acreage size." The evaluator and the installer both have to be certified and registered with the Franklin County Building Department. Sewer permits are issued by that department, not by the health department, and a septic inspection is on the county's own list of construction inspections.
Which building code applies, and what design numbers come with it?
For unincorporated Franklin County it is the 2021 International Residential Code with Appendix Chapters E and J, in effect since 23 April 2024, plus the county's own amendments. The county fills in its own Table R301.2: 20 psf ground snow load, 20 psf roof snow load, 105 mph design wind speed, no ice shield underlayment required, seismic design category C, severe weathering, a 30-inch frost line depth, moderate to heavy termite exposure, slight to moderate decay, a 0 degree F winter design temperature, a mean annual temperature of 56 degrees and an air freezing index of 1,000. That page is the one to hand a foundation and frame quote. If your land is inside a city, ask that city for its own copy — the numbers are genuinely different a few miles apart.
Is Washington, Union or Pacific the same as the county for permits?
No, and this is the most useful thing to know before buying near one of them. A county code adopted under RSMo 64.170 applies to unincorporated territory, so a parcel inside a city is that city's business. Each of the three has adopted its own code on its own date: the City of Washington adopted the 2021 I-Codes with its own Exhibit A of amendments by Ordinance No. 23-13877 on 4 December 2023; the City of Union adopted the 2021 IBC by Ordinance No. 4630 and the 2021 IRC by Ordinance No. 4636, both on 13 November 2023; the City of Pacific is still on the 2018 IRC, adopted by Ordinance No. 3349 on 20 December 2022. Their published design wind speeds are 115 mph, 115 mph and 90 mph against the county's 105 mph. Washington adds a step the others do not: an inspection and certificate of occupancy are required before anyone may occupy a residential structure, with a $50.00 inspection fee for a single-family or two-family dwelling and a $350.00 refundable escrow deposit held until the final certificate issues. Sullivan, St. Clair, New Haven, Gerald, Berger, Leslie, Parkway, Oak Grove Village, Miramiguoa Park and Charmwood each set their own rules too, and Pacific and Sullivan both straddle a county line.
Which zoning district will my land be in, and how big does the lot have to be?
Most rural Franklin County land is in the "W" district, which the Unified Land Use Regulations describe as the district for agricultural and small-scale commercial uses mixed with low-density residential. Do not read the letters the way you would elsewhere: in this county "A" is a commercial, retail, service and manufacturing district, not an agricultural one, and "R" is highway commercial. In W, single-family dwellings are a permitted use, an un-subdivided lot may carry up to two site-built or modular dwelling units at a maximum density of one single-family dwelling unit per three acres, and the minimum lot area for a subdivision without central water and sewer is three acres. That three-acre floor without central utilities repeats across every rural district in the regulations. Setbacks are assigned per district by Planning and Zoning on the application itself, so ask for them in writing at the sketch-plan stage. The regulations are not posted online in full; the department supplies copies on request, at (636) 583-6369.
Does the county have rules for post-frame and steel-frame buildings specifically?
It has more of them than most. Franklin County's residential amendments carry a post-height capacity table, Table R602.7(1), running from a 4x6 at 8 feet through 6x6 posts and utility poles to built-up 3-, 4- and 5-ply columns at heights to 22 feet, and a pole barn header chart, Table R602.7(1)(1), for post spacings of 8 to 16 feet and clear spans of 20 to 40 feet at 20 lb live load and 5 lb dead load with purlin and metal roofing. The footnotes set the concrete under each post, require every header to bear on a post or a bearing block at least 20 inches long and be bolted or lagged to it, and size roof purlins to 2021 IRC Table R802.4.1(1). A Registered Design Professional's design is required for buildings over 5,000 square feet or anything outside the chart. For steel frames there is no chart: the county's permit guide states that steel-frame construction requires a Missouri Registered Design Professional's seal on the building plans, and the same applies to suspended floors spanning more than 7 feet 4 inches.
What does the permit itself cost, and what else has to be paid?
The county publishes a fee rate schedule and a set of rules around it. A non-refundable $25.00 application and processing fee is due when the application is filed; the minimum calculated construction permit fee is $150.00; escrow is paid on a separate check; permits must be paid in full before the first building inspection; and if construction has started before the permit is obtained, the permit fee is doubled and sealed as-built plans from a Missouri Design Professional may be required. Separately, a driveway or entrance permit onto a county road is $60.00 and so is any other work in the roadway right-of-way, with the homeowner buying and installing the culvert. On the zoning side, a conditional use permit application is $650.00 and a rezoning request is $750.00, each due at least thirty days before the Planning and Zoning Commission meeting, which is held on the third Tuesday of the month. Confirm the current schedule with the department before you rely on a number — a fee table is the thing most likely to have moved.
The county says seismic design category C, but I have seen D for this part of Missouri. Which is right?
Both, for different purposes, and the honest thing is to carry both. Franklin County's adopted table says C, and so do the tables of Washington, Union and Pacific — that is the figure a permit is issued against, because the residential code derives the category by its own route from the map current when that edition was written and then freezes. Query the USGS design-map service at a point in the middle of the county for Risk Category II at Site Class D and it returns Ss 0.324 g, S1 0.142 g and category D under ASCE 7-16, and category D again under the newer ASCE 7-22. Neither source is in error. Use the adopted C for the permit, and put both in front of whoever seals the structural design, because the difference shows up in connection detailing and in the lateral system rather than in the roof. It is also worth knowing that Missouri's own seismic statute, RSMo 319.200 to 319.207, does not reach a house at all — section 319.203.2 exempts any single-family or duplex residence and any private structure under 10,000 square feet. What reaches your building is the locally adopted code, and nothing else.

Building in Franklin County?

Start with the plan. Tell us what you want to build and we will draw it, then confirm which office issues the permit for your parcel, what it will let you put there, and what wind load it has to meet. That conversation costs nothing.

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