Jefferson County County-adopted code, county inspection

Barndominium Builders in Jefferson County, MO

Start with the thing that wastes more of people's time here than anything else: this page is about Jefferson County, the charter county on the Mississippi immediately south of St. Louis County, with its seat at Hillsboro. It is not Jefferson City, which is the state capital, sits in Cole County about ninety miles west, and answers to an entirely different building department. If the land you are looking at is near Arnold, Festus, De Soto, Hillsboro, Cedar Hill, High Ridge, Imperial, Byrnes Mill or House Springs, you are in the right place. There is real acreage here — the 2022 Census of Agriculture counted 656 farms on 106,429 acres, a 162-acre average, and 36,706 of those acres are woodland, which is what the northeast corner of the Ozark Highlands looks like in a table. And unlike the county to the north, most people here live outside a city: 166,008 of the county's 231,888 residents live in the unincorporated balance, which is exactly the territory the county's own code governs. That code is real and it is enforced. Jefferson County adopted the 2015 International Residential Code, it prints its own filled-in design table with a 30-inch frost line, and its zoning puts a five-acre minimum under a dwelling in the RA-5 Rural/Agricultural District — but then allows up to 4,000 square feet of detached accessory building, or five percent of the unbuilt lot area, whichever is more permissive. Two things will decide more builds here than the code does. The first is the septic ground: the county runs its own On-Site Sewage Treatment Code, it applies to every system in unincorporated territory, it bars a conventional absorption field on slopes over 20 percent, and it wants three feet of soil between the field and bedrock — which on a cherty hillside is the whole question. The second is the Old Lead Belt: EPA's Southwest Jefferson County Mining Superfund site takes in all of Jefferson County except Herculaneum, and EPA will sample a yard for lead at no cost to the owner. For a reader comparing where to buy, the county line matters: St. Louis County to the north is another charter county with its own code, Franklin and St. Francois are first-classification counties, and Washington and Ste. Genevieve to the west and south are third-classification counties with no county building-code authority at all.

Jefferson County at a glance

County: Jefferson County
Population: 231,888
ZIP codes: 63010, 63012, 63016, 63019, 63020, 63023, 63028, 63047, 63048, 63049, 63050, 63051, 63052, 63053, 63070
Site visits: Usually the same week — the office is in Jennings, a straight run down I-55

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Building conditions in Jefferson 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 Jefferson 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.

This is Jefferson County, not Jefferson City

Two different places, and confusing them is the most common mistake made about this market. Jefferson County is a charter county on the Mississippi south of St. Louis County; its seat is Hillsboro, and its towns are Arnold, Festus, Crystal City, De Soto, Herculaneum, Pevely, Byrnes Mill and Hillsboro, with Cedar Hill, High Ridge, House Springs and Imperial as unincorporated communities the Census Bureau classes as census designated places. Jefferson City is the state capital, and it is in Cole County, about ninety miles west of Hillsboro in a straight line. They have different building departments, different adopted codes and different zoning. Nothing on this page applies to Jefferson City.

Home Rule Charter of Jefferson County, Missouri § 1.4 (county seat), read via the county's codified ordinances on eCode360 (Jefferson County, MO, code JE3328, current through 13 July 2026) 6 September 2026; municipality list and populations from U.S. Census Bureau Vintage 2024 Subcounty Population Estimates, sub-est2024_29.csv, SUMLEV 157 records for county FIPS 29099; place classification and coordinates from the 2024 Census Gazetteer places file; straight-line distance computed from the Gazetteer internal points for Hillsboro city and Jefferson City city — both files fetched 6 September 2026

The authority is the county's charter, not RSMo 64.170

Jefferson County is one of Missouri's five charter counties, and a charter county sits outside the four-class system that decides whether an ordinary Missouri county may have a building code at all. Its Charter says where the power comes from: except as the Charter or the state constitution limits it, the county has "all the powers of a county of the State of Missouri of whatever class and all the powers of all offices, agencies and departments of counties of the State of Missouri of whatever class." That is why the section a reader will find quoted everywhere else about Missouri county building codes — RSMo 64.170, which reaches only counties of the first and second classification — is the wrong citation here, even though the Revisor's own catchline for it still carries the words "Jefferson County, separate provision" from an older version of the section.

Home Rule Charter of Jefferson County, Missouri §§ 1.5 and 1.6, read via eCode360 (Jefferson County, MO, JE3328) 6 September 2026; Missouri Association of Counties, "Missouri Counties by Classification", published May 2025, CHARTER list of 5 counties, PDF read 6 September 2026; Mo. Const. Art. VI §§ 18(a) and 18(c); RSMo 64.170 catchline and text, effective 28 August 2008, rendered from revisor.mo.gov 6 September 2026

The county fills in its own design table, and here is the whole row

Jefferson County prints a completed Table R301.2(1) in the ordinance itself: ground snow load 20 lbs/ft²; wind speed 115 mph (51 m/s); topographic effects NO; special wind region NO; wind-borne debris zone NO; seismic design category C; weathering SEVERE; frost line depth 30 inches; termite NO; winter design temperature 6 °F; ice barrier underlayment required NO; air freezing index 1,000; mean annual temperature 55.2 °F. The 30-inch frost line is the number that sets the depth of every pier and every post hole on a post-frame building, and it is the figure to hand a foundation quote. The 6 °F winter design temperature is the county's own and is colder than some neighbours publish, so do not carry a heating calculation across the county line.

Jefferson County Code § 500.400, Article III, Amendments to the International Residential Code 2015, Table R301.2(1) as adopted (Ord. No. 11-0248 § 1(Art. III § 1), 9 August 2011; Ord. No. 13-0459, 10 December 2013; Ord. No. 17-0431, 28 September 2017; Ord. No. 23-0465, 16 November 2023), read from the rendered code on eCode360 6 September 2026

Five acres for the house, but 4,000 square feet of shop

In the RA-5 Rural/Agricultural District — the district that covers the county's open land — a dwelling needs a five-acre tract, and the density cap is one unit per five acres. In exchange, the accessory-building allowance is generous by metropolitan standards: in RA-5 and LR-2 the combined gross floor area of accessory buildings may reach 4,000 square feet or five percent of the lot area not occupied by the primary use, whichever is most permissive. On a five-acre parcel that five percent test is worth roughly ten thousand square feet, so the 4,000-square-foot floor is not usually the binding number. One catch worth planning around: on a lot where the primary building has not yet been constructed, the total of all accessory structures is capped at 4,000 square feet until the house exists. Setbacks in RA-5 are 100 feet from a major street, 65 feet from a county-maintained street, 25 feet from any other street, 20 feet at the sides and 50 feet at the rear.

Jefferson County Code § 400.2870 (Zoning Order § 8.060, 2 April 2008; Ord. No. 07-30-2008A §§ 1–2(8.060), 30 July 2008), § 400.1470, § 400.1660 Table 5-2 and § 400.1670 Table 5-3 (Ord. No. 16-0100 § 2, 4 January 2016), read from the rendered Unified Development Order on eCode360 6 September 2026

The county runs its own septic code, and the three-acre rule does not clear you here

Jefferson County has its own On-Site Sewage Treatment Code, and it applies to any on-site sewage treatment system located within the unincorporated land of the county — with no acreage exemption in it. That matters because the exemption people arrive quoting, the one for a single-family lot of three acres or more, is an exemption from the state programme in RSMo 701.031, and RSMo 701.047 lets a county set standards more restrictive than the state's. The county's rules are where a cherty hillside gets expensive: a conventional soil absorption system may not sit on land sloping more than 20 percent and must stand at least 20 feet back from the crown of such a slope; there must be three feet of soil between the bottom of the field and high groundwater or bedrock, and 48 inches of suitable soil for a conventional system. Soil borings are required on every site regardless of system type — a minimum of four for new construction, one of them made with a backhoe, plus two more in the reserve area — and the design must be prepared by a designer the county licenses.

Jefferson County Code §§ 705.010, 705.020(A), 705.170, 705.180 and 705.210 (Ord. No. 11-0248 § 1, 9 August 2011), and § 500.130 On-Site Sewer System Designer License, read from the rendered code on eCode360 6 September 2026; RSMo 701.031 and 701.047 as recorded in research/statewide/code-regime.md § 7a

The whole county sits inside a Superfund site, and EPA will sample your soil free

EPA's Southwest Jefferson County Mining National Priorities List site "includes all of Jefferson County, Missouri, excluding the Herculaneum Site." The county is on the northern edge of the Old Lead Belt, where lead mining began in the early 1700s and produced more than eight million tons of lead and an estimated 250 million tons of mine waste. Mine waste travelled downstream on the Big River and was also hauled and spread by hand: EPA records it turning up on residential properties as topsoil, as fill material and as road aggregate. The site was listed on the National Priorities List on 23 September 2009 and is divided into eight operable units; OU-4 is the Big River and its floodplain. The remedy for residential yards includes excavation and replacement of soil above 400 parts per million lead, and institutional controls specifically aimed at safeguarding future residential development and protecting cleaned properties from recontamination. Sampling of residential yards continues at no cost to the property owner. Being inside the site boundary is not the same as having contaminated soil on your parcel — but on rural land near the Big River it is a question to answer before the slab goes down, not after.

U.S. EPA Superfund Site Profile, SOUTHWEST JEFFERSON COUNTY MINING, Jefferson County, MO — Cleanup Activities page (cumulis.epa.gov, site id 0705443), fetched and read 6 September 2026; U.S. EPA Region 7, "Superfund National Priorities List (NPL) Sites Near Big River and Floodplain; St. Francois, Jefferson, and Washington Counties, Missouri – Fact Sheet, September 2021" (epa.gov/mo), fetched 6 September 2026

Also worth knowing before you build in Jefferson County

Karst is not this county's problem — 54 mapped sinkholes against 1,474 next door: The reflex about the Ozark border is sinkholes, and here the data says otherwise. Missouri's Department of Natural Resources publishes a statewide layer of known and probable sinkhole locations holding 16,001 points, which matches the roughly 16,000 the department publishes in prose. Queried against the county boundary, 54 of those points fall inside Jefferson County. St. Louis County, immediately north and far more built up, holds 1,474; Greene County around Springfield holds 1,471; St. Francois County to the south holds 133. What shapes a foundation here is slope and shallow rock on wooded hillsides, not solution cavities — which is why the county's own septic code writes rules about 20 percent slopes and three feet of soil above bedrock rather than about sinkhole setbacks.
New flood maps took effect in October 2025, and the county rewrote its ordinance for them: Jefferson County repealed its entire Flood Damage Prevention chapter and replaced it on 4 December 2025. The new chapter runs against a Flood Insurance Study for Jefferson County dated 2 October 2025 and Flood Insurance Rate Map index panels 29099CIND1C and 29099CIND2C of the same date, and it requires a separate floodplain development permit for any development in a mapped A, AE, AO or AH zone. It also draws the line a barndominium buyer needs to see: a structure used solely for agricultural purposes may be built at grade and wet-floodproofed only where "there is no human habitation or occupancy of the structure," it is of single-wall design, and a floodplain development permit has been issued — and even then a variance is required whenever the lowest floor is at grade below the base flood elevation. A house on a farm is not an agricultural structure.
A driveway onto a county road is its own permit, with a refundable escrow: Since 26 November 2024 anyone building, modifying or reconstructing a private driveway entrance onto a county-maintained road in unincorporated Jefferson County has to apply to the Department of Public Works for a permit. The escrow is set by driveway type: $500.00 for a single-lot residential entrance 12 to 28 feet wide, $500.00 for an agricultural gravel entrance 16 to 40 feet wide, $1,000.00 for a residential subdivision entrance and $1,500.00 for a commercial or industrial one. The county cashes the escrow and holds it as a bond, keeps whatever it needs to repair damage in the right-of-way, and returns the balance when the entrance is accepted. Design standards start with a 125-foot minimum offset, centre to centre, from any other entrance or intersection on the same side of the road — which on a narrow-frontage parcel can decide where the driveway goes before the house is sited.
The county licenses the trades, and tests homeowners who want to do their own: There is no general or building contractor licence in Jefferson County. What Chapter 500 licenses is plumbing, electrical, mechanical, communications, sprinkler fitting, lawn irrigation installation, on-site sewer system installation, on-site sewer system design and on-site soil evaluation — the trades, not the builder. The plumbing licence provision goes further than most and reaches advertising: no contractor may perform plumbing work "or hold itself out in any advertising medium or publication as being available to perform plumbing work" without it. A property owner may take out a permit to do electrical, plumbing, mechanical or drainlaying work on their own detached single-family dwelling, but only if they occupy it, personally perform all the work, hold no other open permit, and pass a written examination at 75 percent or better; two failures and the applicant waits a year.
The permit fee is priced off the county's own square-foot valuation: Jefferson County does not ask what your builder charged. Section 500.250 sets the minimum cost of construction the county will use to calculate a permit fee — $120.00 per square foot for new residential work, $38.00 per square foot for utility buildings, $165.00 for residential remodelling and $65.00 for a finished basement — and then reads the fee off a bracketed table. A build valued between $300,001 and $320,000 draws a $1,725.00 building permit fee. Residential plan review is $40.00 per submittal and each revision, the planning and zoning fee is $10.00, the processing fee is $25.00, and an inspection or re-inspection is $50.00. The split between the $120.00 and the $38.00 rate is worth reading twice on a shop-and-house: the county prices the two halves of the building differently.
The sprinkler offer is a submittal document here: Missouri does not let any city or county make residential fire sprinklers mandatory in a new one- or two-family dwelling, but it does require the builder to offer them at the purchaser's cost before the purchase contract is signed. Jefferson County turns that state duty into a piece of paper the building department will not proceed without: its amendment to IRC R106.1 provides that submittal documents shall include the form, signed by the purchaser of a single-family dwelling or a multi-unit dwelling of four or fewer units, stating that the option was offered. The same amendment requires construction documents for a new one- or two-family dwelling to carry the name and contact information of whoever prepared them.
The Charter makes the county review its own codes every three years: Missouri imposes no currency requirement on an adopting county — RSMo 64.196 asks only for a nationally recognised code of 1999 or later, with no review cycle and no state approval. Jefferson County imposed one on itself. Charter § 6.3.1.3 requires the Code Commission to review all building codes and related ordinances whenever the underlying model code is revised, "but in any event once every three years," and to propose any modification or repeal it thinks necessary, after making a finding on the public benefits and on the costs of enforcement and compliance "including increases in costs of construction." A review is not an adoption: the county has run the 2015 editions since it moved to them, and the codes it adopted in 2011 are still the frame. But it is a rare thing in this state for the question to be on a calendar at all.
656 farms, 106,429 acres, and a third of it woodland: The 2022 Census of Agriculture counted 656 farms in Jefferson County on 106,429 acres, an average holding of 162 acres, in a county with 656.264 square miles of land. Of that land in farms, 38,891 acres are cropland, 25,366 acres pastureland and 36,706 acres woodland — better than a third of it wooded, which is the Ozark Highlands showing up in a table. The size distribution is what a buyer actually wants: 60 farms are between one and nine acres, 221 between ten and forty-nine and 245 between fifty and 179, so roughly four in five holdings here are under 180 acres. Land in farms rose 17 percent between 2017 and 2022 while the number of farms fell 9 percent, which is consolidation rather than the subdivision happening in the county to the north.
Two of the counties on this one's border cannot have a building code at all: Jefferson County touches six others — St. Louis County to the north, Franklin to the northwest, Washington to the west, St. Francois to the southwest, Ste. Genevieve to the south, and Monroe County, Illinois across the Mississippi. Under the classification list published by the Missouri Association of Counties in May 2025, St. Louis County is the other charter county, Franklin and St. Francois are first-classification counties, and Washington and Ste. Genevieve are third-classification counties. That last word is the one that changes an answer: RSMo 64.170 empowers only counties of the first and second classification to adopt a building code, so a third-classification county has no county building-code authority whatsoever, and no county building department is going to inspect the work. Crossing this county's southern or western line can therefore change not just the code but whether there is one.
The Big River carries the Old Lead Belt into the Meramec here: The Missouri Department of Conservation puts the Meramec basin in "the northeast corner of the Ozark Highlands," draining 2,149 square miles across ten counties including this one, with base flows sustained by springs and by two major tributaries — the Bourbeuse, which enters at river mile 64.0, and the Big River, which enters at river mile 35.7. That second confluence is the one that matters here: EPA describes the Big River as running through the Old Lead Belt and carrying mine waste downstream onto the floodplain, and it has made the Big River and its floodplain a distinct operable unit of the Superfund site that covers this county. So the two facts a buyer needs about water here — the Meramec draining a wooded Ozark watershed, and the Big River arriving from the lead district — are the same river system.
The corrugated-steel ban in this county's code is a data-centre rule, and it stops there: Jefferson County's Unified Development Order does carry a prohibited-materials list, and corrugated steel is the first item on it, so it is worth being exact about what that list reaches. Section 400.3700(N)(1) provides that buildings and accessory structures "shall not include the following as primary building materials or architectural finish: a. Corrugated steel. b. Smooth finish concrete block (cinder block). c. Smooth finish poured concrete, whether painted or unpainted. d. Vertical rough-sawn wood siding (T-111/T-19). e. Tensile fabrics. f. Single layer uninsulated materials." That section sits inside Article X, Data Center Regulations And Performance Standards, added by Ordinance No. 26-0203 on 27 April 2026, and the article states its own reach one section earlier: Section 400.3670(A) says "These standards apply to all Data Centers, whether principal or accessory use." It does not touch a dwelling. The design-standards article says the same thing from the other direction. Section 400.2170(C) applies those standards to all zoning districts "with the exception of the 'RA-5' Rural/Agricultural District", which is the district almost all rural acreage in the county sits in, and Section 400.2200 sends the rural and rural-cluster patterns back to the RA-5 district standards, which govern density, lot size, setbacks and height and say nothing about what a building is clad in. The suburban and mixed-use residential design standards at Sections 400.2250 and 400.2310 are street layout, sidewalk and lot-access rules. Nowhere in the Order is there a minimum roof pitch on a house, a minimum eave projection, a minimum masonry percentage or a minimum dwelling floor area, and the four-sided-architecture clause at Section 400.2320(A)(3)(c) is a transition standard between residential and non-residential uses in the mixed-use pattern, written as "similar architectural details, materials and colors" rather than as a material list. Chapter 500, which adopts the residential code and amends it at length, carries no exterior-covering provision either. Appearance review exists in one place: the determination of appropriateness at Section 400.2020, required before construction affecting a designated landmark or a property inside a nominated or designated historic district. Inside their own limits the county's municipalities answer for themselves.

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 Jefferson County

All nine services are available in Jefferson 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.

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What a build looks like

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

Stage one

The parcel

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

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.

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 Jefferson County. Tell us what you want to build and we will draw it to them.

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Where we build around Jefferson County

Jefferson 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 Jefferson 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 Jefferson 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.

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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

Franklin County

St. Francois County

Building in Jefferson County: common questions

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

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Is this the same as Jefferson City?

No, and they are not close. Jefferson County is a charter county on the Mississippi River immediately south of St. Louis County, with its seat at Hillsboro and its population centres at Arnold, Festus, Crystal City, Herculaneum, Pevely and De Soto. Jefferson City is the state capital and it lies in Cole County, about ninety miles west of Hillsboro in a straight line. Different county, different building department, different adopted code, different zoning. If you have been reading permit rules for Jefferson City and your land is in Cedar Hill or House Springs, none of it applies to you.

Is there a building code here, and who inspects?

Yes, and the county does. Jefferson County adopted the 2015 International Residential Code — along with the 2015 IBC, fire, fuel gas, mechanical, energy, plumbing and swimming pool codes and the 2014 National Electrical Code with the county's own 2017 electrical code — at County Code § 500.010, first by Ordinance 11-0248 on 9 August 2011 and amended since. The Code Official is the Director of County Services and Code Enforcement, and the Code Enforcement Division does the plan review and the inspections. Two limits are worth knowing. The county code reaches unincorporated territory only, under Charter § 6.3.1.7, so inside Arnold or Festus or any of the other municipalities the answer is that city's. And the county does not adopt the model codes whole — Chapter 500 carries eleven articles of amendments, including one that deletes chapters of the energy code. Read the article, not just the edition year.

Can I put up a separate shop building next to the house?

In the RA-5 Rural/Agricultural District, yes, and with room to spare. Section 400.2870 allows accessory buildings in RA-5 and LR-2 up to a combined 4,000 square feet or five percent of the lot area not occupied by the primary use, whichever is most permissive — and on a five-acre parcel the five percent test is the more generous of the two by a wide margin. There are two things to plan for. Until the house is built, everything accessory on the lot together is capped at 4,000 square feet. And a detached accessory structure of 200 square feet or less needs no building permit under the county's amendment to IRC R105.2, which is not the same as saying it escapes the zoning rules — the setbacks and the accessory-area cap still apply, and an accessory building of 200 square feet or less has to sit at least 15 feet off the rear property line.

How much land do I need, and where does the building have to sit?

In RA-5, five acres per dwelling, with a five-acre minimum lot and 150 feet of lot width on a major street or 50 feet on any other. The alternative is the open space conservation route, which lets the minimum lot drop to one acre but holds density to 0.28 units per acre across the tract. Setbacks in RA-5 and LR-2 are 100 feet from a major street, 65 feet from a county-maintained street, 25 feet from all other streets, 20 feet at the side and 50 feet at the rear — and the front setback may instead be set by the line of existing homes on the same side of the street. Those are county numbers for unincorporated land only; inside any of the county's municipalities the district and the minimums are that city's.

My lot is over three acres — do I still need a septic permit?

Almost certainly yes, and this is the single most expensive assumption people bring to rural Missouri. The three-acre exemption is written into RSMo 701.031 and it excuses the owner from the state's on-site sewage provisions — but RSMo 701.047 lets a county impose standards more restrictive than the state's, and Jefferson County has. Its own On-Site Sewage Treatment Code applies to any on-site sewage treatment system located within the unincorporated land of the county, and no acreage threshold appears in its applicability section. Expect soil borings on the site regardless of the system type — four for new construction, one of them with a backhoe, plus two in the reserve area — a design prepared by a designer the county licenses, and verification of soil texture, mottling, groundwater and bedrock depth by an on-site soil evaluator.

The land I like is a wooded hillside. Will it perc?

That is the right worry in this county, and "perc" is not quite the test. Jefferson County's septic code sets the limits directly: a conventional soil absorption system may not be located on land with a slope greater than 20 percent, and must sit at least 20 feet back from the crown of such a slope. There must be a minimum of three feet of soil between the bottom of the absorption system and high groundwater or bedrock, and 48 inches of suitable soil for a conventional system, with soil loading at 0.4 gallons per square foot or faster. On the cherty, shallow-to-rock hillsides that make up much of the western and southern half of the county, that soil depth is what fails a site, not the slope alone. Where a conventional field will not work the code provides for alternative and innovative systems — but that is a design decision and a cost, so get the borings done before the contract closes, not after.

The county says seismic design category C, but I have seen D. Which governs?

Both are correct, for different purposes, and the honest answer is to carry both. Jefferson County's own adopted Table R301.2(1) says category C, and the permit is issued against that, because the IRC derives its category by its own route from the map current when the adopted edition was written and then freezes at that edition. Query the USGS Seismic Design Web Service against ASCE 7-16 at Risk Category II and Site Class D for a point in the middle of the county and it returns Ss 0.459 g, S1 0.172 g and category D. Neither source is wrong. Use the county's C for the permit, and put the current values in front of whoever does the structural design, because the difference shows up in connection detailing and in the lateral system rather than in the roof.

What is the Superfund listing about — is my land contaminated?

The Southwest Jefferson County Mining National Priorities List site takes in all of Jefferson County apart from the separate Herculaneum site, and it was listed on 23 September 2009. The reason is the Old Lead Belt: lead mining began here in the early 1700s, produced more than eight million tons of lead and an estimated 250 million tons of mine waste, and that waste moved downstream on the Big River and was also hauled around the county and used as topsoil, fill and road aggregate on ordinary residential property. Being inside the site boundary is not a finding about your parcel — the boundary is countywide and most of it has never been sampled. What it does mean is that a free answer is available: EPA continues to sample residential yards at no cost to the owner, and the remedy for residential soil is excavation and replacement above 400 parts per million lead. On floodplain land near the Big River, and on any parcel where fill or gravel of unknown origin has been brought in, ask for the sampling before you commit to a footprint.

Do I need a licensed contractor to build here?

Jefferson County issues no general or building contractor licence. What it licenses is the trades — plumbing, electrical, mechanical, communications, sprinkler fitting, lawn irrigation, on-site sewer installation and design, and on-site soil evaluation — and those licences are real, with experience requirements, examinations, security and insurance conditions attached. Missouri issues no statewide general contractor or residential builder licence either; the state's Division of Professional Registration has a board for electrical contractors and none for general contractors, home builders, plumbers or mechanical contractors. If you intend to do part of the work yourself, the county has a route for that: a homeowner permit for electrical, plumbing, mechanical or drainlaying work on a detached single-family dwelling you occupy, conditional on personally performing the work and passing a written examination at 75 percent or better.

Building in Jefferson 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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