ADU Pass
Washington
Warren County portion
Also in: Franklin County
ADU Pass helps homeowners in Washington, Warren County, Missouri navigate the permit paperwork for building an accessory dwelling unit. This area covers 1 ZIP code.
Map
ADU details
ADU legality: with-restrictions
This Warren County cross-listing inherits Washington's city ordinance (Franklin County is the authoritative county). Net effect tracks the Franklin County primary entry. A genuine unincorporated Warren County parcel would be reviewed by Warren County P&Z, not by the city.
Cost scenarios
| Scenario | Sq ft | Permit | Build | Total |
|---|---|---|---|---|
| minimum | 150 | $1,700 | $33,000 | $34,700 |
| 600 | 600 | $1,700 | $132,000 | $133,700 |
| midpoint | 525 | $1,700 | $115,500 | $117,200 |
| maximum | 900 | $1,700 | $198,000 | $199,700 |
Fee breakdown (as of 2026-04)
Permitting process
- Resolve county-of-jurisdiction first (~1d)
Confirm whether the parcel lies inside the City of Washington corporate limits (Franklin County) or in unincorporated Warren County across the Missouri River. The Warren County zoning order reaches only unincorporated Warren County parcels and does not apply inside the city of Washington. - Choose the correct review track (~2d)
If the parcel is unincorporated Warren County, follow the Warren County Planning and Zoning track: pull the General Application Form, sketch plan, and Conditional Use Permit packet from warrencountymo.org. If the parcel is inside the city of Washington in Franklin County, follow the Title V city track via SmartGov. - City track: confirm Title V district (~3d)
Check the parcel's residential district (R-1A, R-1B, R-1D, or R-2) on the City zoning map; the Reorganized Development Code uses Title V Supplementary Regulations to govern accessory uses and structures including conversion of existing structures to dwelling use. - Pull washmo.gov / handouts (~2d)
Pull the City's accessory-structure, room-addition, residential-occupancy, and Development Code handouts off the washmo.gov Handouts page; these define what counter-staff will look for at intake. - Submit through SmartGov + post escrow (~1d)
Open an account on the City of Washington SmartGov public portal, complete the building-permit application with site plan, floor plan, elevations, and post the $350 refundable escrow deposit. - Engineering plan review (2021 IBC family / 2020 NEC) (~21d)
Building Inspections checks plans against 2021 IBC, 2021 IRC, 2021 IPC/IMC/IFGC, and 2020 NEC; structural for Missouri River floodplain parcels gets a separate floodplain check. Warren County floodplain coordination invoked for any parcel touching the Warren-side river edge. - Pay fees and receive issued permit (~3d)
Building permit and any utility-extension fees collected; permit card and approved plans returned via SmartGov for posting on site. - Inspections and Residential Occupancy Permit (~90d)
Foundation, framing, rough-in trades, insulation, drywall, and final; once final passes, Engineering issues the Residential Occupancy Permit which is the local CO-equivalent before any tenant moves in.
Viability (permitted uses)
- Long-term rental: yes Long-term rental of ADU generally permitted; landlord-tenant law and Washington's Residential Occupancy Permit gating apply.
- Short-term rental: with-restrictions Washington regulates STRs separately from ADU permitting; check city STR ordinance and any HOA covenants. Warren County imposes no STR rule on Washington city parcels because the city is not in Warren County.
- Office rental: with-restrictions Detached office rental requires home occupation permit or rezoning under Washington Title V.
- Home office: yes Home occupation permitted with restrictions on signage and customer traffic.
- Studio / workshop: yes Personal artist studio is a permitted accessory use.
- Agriculture: with-restrictions Limited urban agriculture permitted in residential zones; livestock varies by district. Missouri River floodplain parcels carry additional restrictions.
- Relative support: yes Family-occupancy ADU explicitly permitted in single-family zones.
Contacts
Staff: Washington City Hall switchboard (General municipal services / Planning & Zoning routing), Washington Planning & Zoning Division (Zoning Administrator (Title V Reorganized Development Code)), Washington Building Inspections (Engineering Department) (Building Permits & Inspections), Warren County Planning and Zoning Office (unincorporated parcels only) (County Zoning Order administration; CUP packet intake at the Warren County Justice Center), Franklin County Planning & Zoning (actual county of jurisdiction; informational only since the city governs) (Franklin County P&Z Department)
Utilities
- Water: City of Washington Water Utility · 30d connect · $4,500
- Sewer: City of Washington Sewer / Wastewater · 30d connect · $5,500
- Electric: Ameren Missouri (regional electric) · 21d connect · $1,800
- Gas: Spire (regional natural gas) · 30d connect · $1,500
Property values & taxes
Construction timeline
Realistic total: best 7mo · typical 10mo · worst 16mo
Financing
State ADU loans:
Insurance impact
HOA prevalence & preemption
Missouri has no HOA-ADU preemption. Washington has moderate HOA prevalence in newer subdivisions on the south side; older downtown stock and Missouri River bluff lots are largely fee-simple.
Regulatory overlays (2)
- flood-zone — FEMA SFHA along the Missouri River corridor between Washington (Franklin County) and the Warren County north-river bank, plus tributaries (Boeuf Creek, Dubois Creek) · +14d · +8% cost
Finished-floor elevation requirements for parcels inside SFHA; flood insurance required for federally-backed financing. (map) - tornado-zone — Eastern Missouri Tornado Alley exposure (St. Louis metro periphery, Missouri River corridor) · +2% cost
ASCE 7-22 wind design 110 mph baseline for Franklin County; Title V does not mandate storm shelters in single-family residential. (map)
Technical envelope (climate & building code)
Climate & energy code
Building code
Amendments:
- Amendment
- Amendment
Legal history (timeline)
Current ordinance: City of Washington Reorganized Development Code, Title V (Zoning) - Franklin County is the actual county of jurisdiction, adopted 2009-01-01, last amended 2024-04-01
- 1995-07-25 — City of Washington v. Warren County (Missouri Supreme Court, 77541-0) (court-ruling)
Missouri Supreme Court decision arising from the City of Washington's airport facility in Warren County. The case is the canonical reference for the city's split jurisdictional footprint.
Effect: Established that the city's airport across the river in Warren County remains city-owned operational land while the city itself remains a Franklin County city for residential / zoning / tax purposes. - 2009-01-01 — City of Washington Reorganized Development Code (Title V Zoning) (city-ordinance)
Washington consolidated its zoning rules into the Reorganized Development Code with R-1A/R-1B/R-1D/R-2 residential districts plus supplementary regulations on accessory uses and structures.
Effect: Established the modern zoning baseline - accessory residential uses are reviewed under Title V supplementary regulations rather than a stand-alone ADU article. - 2019-09-01 — Warren County Updated Zoning Codes (countywide, unincorporated only) (other)
Warren County Commission adopted the updated countywide Zoning Order applying to all unincorporated Warren County land.
Effect: Confirms Warren County zoning has no reach inside the city of Washington (which is in Franklin County); applies only to unincorporated Warren County parcels. - 2021-01-01 — Washington adopts 2021 International Building Codes (city-ordinance)
Washington Engineering / Building Inspections moved its construction baseline to the 2021 IBC family with the 2020 NEC.
Effect: All new dwelling permits including any accessory-dwelling work must conform to 2021 IBC/IRC and 2020 NEC; older Missouri-amendment language for IRC 2018 superseded inside city limits. - 2023-06-01 — SmartGov online permit center launch (city-ordinance)
Washington moved building-permit intake to the SmartGov Community public portal, allowing online application submittal and plan upload in lieu of paper-counter only.
Effect: Reduced counter visits for accessory-structure / dwelling-conversion permits; Engineering still requires the $350 refundable escrow on most building permits.
Known issues (1)
- data-quality (since indefinite) — Some address-classification systems list Washington, MO under Warren County. Washington, MO is in Franklin County; the city only owns operational airport land in Warren County (City of Washington v. Warren County, 1995). Use the Franklin County entry for the authoritative profile. (source)
Missouri state — ADU law and programs
State financing programs
Missouri Housing Development Commission (MHDC), the state housing finance agency under the Department of Economic Development, does not operate an ADU-specific loan or grant product as of 2026-04-26. Construction or rehab of an ADU on a primary residence may be financed indirectly through MHDC's First Place Program (first-time-homebuyer mortgage with below-market rate) or Next Step Program (mortgage paired with a forgivable second of up to 4% of loan amount). Both are mortgage-purchase products and require the property to be the borrower's primary residence; an existing ADU on the property is permitted under standard agency guidelines. MHDC also administers Low-Income Housing Tax Credits, the Affordable Housing Assistance Program (AHAP) tax credit, and federal HOME funds — none of which is a homeowner-facing ADU instrument.
Federal (United States) — ADU-relevant rules and programs
Federal ADU law
The United States has no federal statute that directly regulates accessory dwelling unit entitlement or design. Land-use authority over ADUs resides with states and local governments under the traditional police power. Federal engagement is limited to financing (Fannie/Freddie/FHA/VA/USDA), flood insurance (FEMA/NFIP), and discretionary housing programs (HUD), which are recorded in sibling sections of this file.
Federal financing programs
Federal housing-finance agencies and GSEs set nationwide underwriting rules that govern whether an ADU can be financed, appraised, and counted toward mortgage qualifying income. The relevant actors are Fannie Mae, Freddie Mac, FHA (HUD), VA, and USDA Rural Development.
Federal tax credits
There is no ADU-specific federal tax credit. ADUs may incidentally qualify for existing federal energy-efficiency and clean-energy tax credits when the ADU construction includes qualifying measures.
Federal housing programs
HUD administers several discretionary programs that can fund ADU-related activity at the grantee's election, but none is an ADU-specific program.
ZIP Code
- 63357
Post Office
- 1777 Washington Xing, 63090