Accessory dwelling unit laws and regulations in Missouri

Missouri has no statewide ADU statute, and the 2026 General Assembly session ended without enacting one. Every rule that governs what you can build, where, and how large lives in a city or county zoning code.

Building codes are local too. Under RSMo §64.170 and §67.280, counties and municipalities adopt the International Residential Code by reference on their own schedules, so the same 700-square-foot cottage gets reviewed under the 2015 IRC in St. Louis County and the 2018 IRC in Kansas City (RSMo §64.170; ICC Missouri adoptions).

Start with the planning or zoning department that covers the parcel, and confirm which one that is. A Kirkwood address, a Maplewood address, and an unincorporated St. Louis County address sit in the same county but answer to different codes. For a contrast with how another large state handles the same question, see our guide to California's ADU laws.

Missouri codes recognize the same basic unit types, but they apply different siting rules to each. Kansas City lets an attached ADU sit in the rear or side yard, while a detached one must go in the rear yard only (KC §88-305-15).

Columbia's June 2026 amendment set a 15-foot rear setback for both types and raised the height allowance for a second story over a garage to 25 feet (Columbia Missourian).

• A detached ADU is a freestanding structure, the backyard cottage or "granny pod." It carries the strictest siting rules, including rear-yard placement and separation from the main house, which is 10 feet in St. Louis City. Columbia also imposes a 30% rear-yard coverage cap.
• An attached ADU shares a wall with the main house. Columbia's attached rear setback dropped from 25 feet to 15 feet in June 2026.
• A garage conversion turns existing floor area into a dwelling. St. Louis City allows an ADU to be built into or on top of an existing nonconforming structure. It can also use the structure's foundations (St. Louis ADU information).
• A basement conversion creates a unit inside the existing house; ask your planning department whether it counts as attached and how the size cap is measured.
• A park model falls under separate state rules. RSMo §700.010 defines a recreational park trailer as an ANSI A119.5 unit under 400 square feet in setup mode, and §700.060 excludes it from the manufactured-home rules (RSMo §700.010). No jurisdiction in this review treats one as a permanent ADU.

The four largest cities' ADU codes check the same six items, and the numbers stay within a fairly tight band. Owner-occupancy is the one that swings.

• St. Louis City allows ADUs in all residential districts; Kansas City on any lot where a detached house is the principal building; Springfield in all single-family districts; Columbia in R-1, R-2, and R-MF by right since June 15, 2026. Unincorporated counties are far narrower (see the comparison below).
• Size caps run from 750 to 1,200 square feet and are usually the lesser of a percentage of the main house and a fixed number (St. Louis City instead uses the greater of 50% or 750 square feet, up to 1,200). Kansas City also caps the footprint at 25% of the rear yard.
• Side setbacks of 3 to 6 feet are typical; rear setbacks run 5 to 15 feet for detached units. Springfield's application form adds a one-hour fire rating for any wall within 5 feet of a lot line (Springfield application form).
• Kansas City and Columbia stop at the lesser of the main house's height or 24 to 25 feet; St. Louis City sets a flat 25 feet, and Springfield's January 2026 form lists 16 feet to the peak.
• St. Louis City and Kansas City require no additional off-street parking space. Columbia requires one for a three-bedroom ADU; Springfield requires one per unit and two for three or more bedrooms (Springfield News-Leader).
• Kansas City and Springfield require the owner to live in one of the two units. St. Louis City and Columbia do not. Kansas City goes further and makes you record a deed restriction before it issues a Certificate of Occupancy.

Lot size is a seventh gate in two cities. Columbia requires 5,000 square feet in every residential district, and Springfield requires a lot larger than 6,000 square feet.

An ADU no longer requires a public hearing in St. Louis City or Kansas City. The same has applied in Columbia since June 2026. These jurisdictions permit ADUs by right, so the approval path runs through a building permit.

The sequence runs roughly the same everywhere:

1. Confirm zoning in writing. St. Louis City sells a zoning letter for $25 (zoning fee schedule).
2. Prepare drawings. Springfield requires site and foundation plans, plus an elevation plan, for any accessory structure of 400 square feet or more; anything over 100 square feet needs a building permit at all (Springfield accessory structures).
3. Meet the local IRC edition. Kansas City and St. Louis City are on the 2018 IRC. Springfield is too. Columbia moves to the 2024 IRC for anything submitted on or after October 1, 2026; earlier submissions stay on 2018 (Columbia code adoption dates). Springfield is targeting November 1, 2026 for the same move.
4. Pay the permit fee. St. Louis City charges $25 at filing plus $10 per $1,000 of construction value over $3,000, with the balance due at final approval (St. Louis permit fees). Columbia charges $2.25 per $1,000 of value plus a 50% plan-review fee, valuing finished space at $165.67 per square foot (Columbia fee schedule). Kansas City uses a downloadable valuation calculator with no published flat rates.
5. Clear inspections and the occupancy step. St. Louis City charges $120 for a residential occupancy inspection. Kansas City will not issue a Certificate of Occupancy until you show proof the deed restriction is recorded.

Missouri does not mandate sprinklers: RSMo §67.281 requires builders of one- and two-family dwellings to offer sprinklers at the buyer's cost (RSMo §67.281). Energy rules are contested but unchanged. HB 2384 would have capped local energy requirements at the 2009 IRC/IECC standards; the House adopted a committee substitute, but the bill was placed on the Senate informal calendar on May 7, 2026 and never reached the Governor. Your city's current energy code still governs.

Timelines vary more than fees do. Columbia's published plan-review guidance lists a 10-working-day initial review and 5 working days for each resubmission.

Kansas City's Certified Permitting Program, launched in late July 2026 for new one-family residential permits, targets 4 business days. Its resubmission target fell from 14 days to 3 (KC City Planning).

St. Louis City averaged 22 days to issue a permit in the 2026 partial year. It missed its 80%-within-30-days goal for three straight quarters after applications jumped 25% when the online portal launched in February 2026 (St. Louis CityStat).

The table below covers the four cities with by-right ADU codes plus two unincorporated counties that show how quickly the rules fall away outside city limits:

Sources: St. Louis ADU information; St. Louis County Planning Commission, April 6, 2026; KC §88-305-15; Columbia UDC; KOMU on Columbia's June 2026 changes; Springfield ADU release; Jackson County ADU ordinance.

Two of Springfield's official documents disagree on setbacks. The Accessory Structures page under §36-450 says 10 feet from the rear lot line, while the residential accessory structure application form dated January 12, 2026 says 3 feet from all lot lines. They may describe different structure types; ask Building Development Services which figure governs a habitable unit before you draw plans.

The unincorporated counties around those cities are a different world:

• Boone County opened up in 2026. Its Planning and Zoning Commission approved a package of affordable-housing code changes on March 19, 2026, and the zoning ordinance now carries a "Last revised March 31, 2026" date. Section 7.1.3 allows a single accessory dwelling on any legally created parcel in the A-1, A-2, A-R, and R-S districts, capped at the lesser of 75% of the primary dwelling or 1,500 square feet. It requires 10 feet of separation unless the two share a common wall, one extra off-street space if the unit has more than three bedrooms, and common ownership of both dwellings. A Columbia-area lot outside city limits can therefore have a real ADU path, and at 1,500 square feet the county cap is larger than any of the four cities above.
• St. Charles County's UDO §405.405.A caps every lot at one single-family dwelling with no ADU exception. Detached accessory structures up to 1,200 square feet on lots under an acre are allowed only as incidental buildings, never as dwellings (St. Charles County UDO).
• Greene County outside Springfield permits an "Accessory Living Area" (guest house or servant's quarters) up to 1,000 square feet with a 10-foot rear setback and 10 feet of separation from the main house. It scales the cap down by lot size: a lot under 7,000 square feet gets 500 square feet (Greene County Article III). Its UR-1 district allows accessory apartments only inside owner-occupied single-family homes.

Inside St. Louis County, the municipalities write their own codes. Kirkwood permits ADUs in R-1 through R-7 with owner-occupancy, a 750-square-foot cap, two bedrooms, and a 15,000-square-foot minimum lot (Kirkwood code). Maplewood allows them as a conditional use in the SR district with an 800-square-foot cap and a ban on short-term rental use (Maplewood ADU regulations). Webster Groves prohibits using an accessory building as living quarters for non-family members and bars rental use outright (Webster Groves Chapter 53).

None of Missouri's major cities or the counties covered here lets a park model RV or a tiny home on wheels serve as a permanent ADU on an ordinary residential lot. The wheels are the problem, and so is the certification.

The RV Industry Association's own policy says park model RVs "by definition, are not meant to be affixed to real property" and that "RVs are not permanent or residential housing" (RVIA). Local codes follow that line:

• Kansas City's §48-39 declares living in a vehicle, RV, or trailer in a residential zone a nuisance. The city adopted IRC Appendix Q for tiny houses, but Appendix Q requires permanent footings under §R403, and the nuisance rule controls anyway (KC Chapter 48).
• St. Louis City's Ordinance 72036 definition of an ADU does not include wheeled units, and the city adopted the 2018 IRC without Appendix Q (St. Louis Ordinance 70795).
• Columbia bars using any vehicle for living or sleeping for more than two consecutive weeks while it is stored on the premises (Columbia).
• Springfield permits storing an RV as long as nobody lives or sleeps in it, and its Appendix Q adoption requires permanent footings below a 24-inch frost line (Springfield 2018 IRC).
• Jackson County's Ordinance No. 5912 (December 16, 2024) expressly prohibits wheeled or portable units as either a principal or secondary residence (Jackson County).
• Unincorporated Greene County allows park models under 400 square feet only inside approved travel trailer parks (Greene County rules).

Sedalia regulates foundation-built tiny homes: its Ordinance 12135 (September 23, 2024) defines a tiny home as 350 to 649 square feet and requires a permanent masonry or concrete foundation. Golden City regulates anything on a chassis or wheels as a mobile home and requires the axles and wheels removed before it can sit on a foundation (Golden City). The pending SB 1774/HB 3230 would stop local governments from banning modular or manufactured homes, but it does not authorize park models as ADUs and it is not law.

Sewer and water connections can add four figures to a Missouri ADU before you pour a footing, and on a septic lot they can stop the project.

• In Kansas City, a 3/4-inch water service line serves one unit only; a 1-inch line can serve two. If your existing service is 3/4-inch, the ADU triggers an upgrade or a second tap. KC Water's 2026 schedule lists a small tap at $303 (3/4-inch iron) to $351 (1-inch PVC), a meter set at $313 (5/8- or 3/4-inch) or $394 (1-inch), a $100 water-service permit, and a $142 sewer connection and inspection fee (KC Water 2026 rules).
• The St. Louis side costs more. The Metropolitan St. Louis Sewer District charges a $1,126 connection fee for a single-family dwelling, and an additional water tap or upsized tap triggers an additional sewer connection fee (MSD connection fees). Add a $75 house connection permit and per-foot inspection charges of $5.80 (sanitary) and $4.20 (storm). Missouri American Water bills new connections at actual cost with no fixed small-service tap fee (tariff).
• Columbia requires a separate meter for each dwelling unit; an owner may elect a single meter but still pays a connection fee for each unit (Columbia Water & Light manual). Columbia Water & Light has not published a one-time connection fee amount; ask when you apply. The monthly base charge is $14.32 for a 5/8- or 3/4-inch meter and $27.46 for 1-inch.
• On a rural lot, the septic system is the deciding factor. Missouri's sizing rule (19 CSR 20-3.060) requires 120 gallons per day per bedroom, so a two-bedroom ADU adds 240 gallons per day of design flow to whatever the existing system was permitted for (19 CSR 20-3.060).

Contractor estimates put a septic expansion at $2,000–$10,000. Drainfield expansion alone runs $3,000–$8,000, while soil or percolation testing costs $700–$2,000 (FreeFlow; Angi).

Permit fees vary by county. Boone County charges $510 for a new system or upgrade and $35 to evaluate an existing system for added bedrooms (Boone County); Jefferson County charges $1,015.35 for a conventional system and $759.15 for a drainfield expansion as of January 2, 2026 (Jefferson County). The state DHSS application fee, formerly $90, is suspended as of August 28, 2026.

A detached new-build ADU in Missouri runs roughly $150 to $360 per square foot, and no Missouri government publishes verified completed-project costs, so every figure below is a builder or calculator estimate. The build path you choose moves the total more than anything else:

Nationally, the Urban Institute puts a typical ADU near $200,000 with a range of $20,000 to $400,000 (Urban Institute), and the NAHB survey of 41 builders found an average of $162 per square foot for new homes generally. Budget permits on top: Columbia's building permit for an 800-square-foot ADU comes to roughly $2,000–$2,400 plus $100 each for mechanical, electrical, and plumbing (Buildet).

The garage conversion is the cheapest legal path in every Missouri city with a published estimate. You reuse an existing structure and footprint, and the four city calculators land within $5,000 of each other.

Missouri offers no state program that funds an individual homeowner's ADU. The Missouri Housing Development Commission's First Place and Next Step programs (manuals revised March 11, 2026) help you buy a primary residence that already has an ADU, treating it as a two-family purchase under the $725,146 non-targeted price limit, and the 4% forgivable down-payment loan cannot fund repairs or construction (First Place manual; 2026 limits). The Missouri Housing Trust Fund accepts only nonprofit or corporate applicants.

Two federal mortgage products do cover ADU construction:

• Fannie Mae HomeStyle Renovation expressly covers building an accessory unit where local zoning permits it, with renovation costs capped at 75% of the lesser of purchase price plus renovation or as-completed value (Selling Guide B5-3.2-01). Fannie Mae allows one ADU on the parcel and lets ADU rent count toward qualifying income, capped at 30% of total qualifying income (Selling Guide B3-3.8-02).
• FHA Standard 203(k) covers ground-up ADU construction. The Limited 203(k), capped at $75,000 and restricted to nonstructural work, does not (HUD ML 2024-13).

USDA guaranteed loans tolerate an existing ADU but disfavor one designed as a rental income stream (USDA HB-1-3555). Outside those products, you are financing with a home equity line or a bank construction loan on your own credit, with no ADU-specific program behind it.

As of September 28, 2026, the two bills most often cited in Missouri ADU searches either never became law or were never about ADUs.

St. Louis Board Bill 43, introduced June 2, 2023, died in committee (Board Bill 43). The law that legalized ADUs citywide is Board Bill 60, passed 12 to 1 on September 26, 2025 and signed by Mayor Cara Spencer as Ordinance 72036 on September 29, 2025, effective the same day (Board Bill 60; Ordinance 72036). Its standards appear in the comparison table above and are in force.

Missouri House Bill 1963 (2026) is titled "Modifies provisions relating to social workers." Rep. Tara Peters sponsored it, the House passed it 151 to 0 on March 30, 2026, and it died in the Senate Emerging Issues and Professional Registration Committee after an April 21, 2026 hearing (house.mo.gov; LegiScan). It has nothing to do with ADUs and never reached Governor Mike Kehoe.

Three 2026 measures do touch ADU builders indirectly:

• HB 2384 would have capped local energy requirements at the 2009 IRC/IECC, loosening the 2018 and 2021 IECC standards Kansas City, St. Louis, Columbia, and Springfield enforce. It cleared the House but died on the Senate informal calendar after May 7, 2026. The "8/28/2026" date attached to it on the legislature's tracker is a proposed effective date, not evidence it passed.
• HB 3466 would create a Missouri Building Codes Act. It is pending, not enacted (HB 3466 summary).
• SB 1774 and HB 3230 would bar local bans on modular and manufactured homes and incompatible foundation requirements. Both are pending (SB 1774).

At the county level, St. Louis County's Planning Commission adopted Resolution No. 01-2026 on April 6, 2026 stating its intent to add ADU rules to Chapter 1003, and P.C. 05-25 (recommended 6 to 1) reached the County Council on January 2, 2026 with proposed accessory-structure caps of 400 square feet or 50% of the primary footprint per structure and a 1,000-square-foot aggregate, plus a definition change allowing "not more than two kitchens" in a dwelling (P.C. 05-25). No final Council passage is confirmed. Check the Council's record before assuming anything has changed for an unincorporated parcel.