ADU Pass
Washington
Franklin County portion
Also in: Warren County
ADU Pass helps homeowners in Washington, Franklin County, Missouri navigate the permit paperwork for building an accessory dwelling unit. This area covers 2 ZIP codes.
Map
ADU details
ADU legality: unclear
Missouri leaves ADU regulation to local municipalities under home-rule or Dillon-rule authority. Washington permits ADUs subject to local conditions per its zoning ordinance.
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
- Confirm zoning district under Title V (~3d)
Check 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. - Review Engineering / Building 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. - Create SmartGov account and submit application (~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. - 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 through final and CO (~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 any city rental-registration ordinance apply.
- Short-term rental: with-restrictions STR rules vary by city. Washington regulates STRs separately from ADU permitting; check local STR ordinance and HOA covenants.
- Office rental: with-restrictions Detached office rental requires home occupation permit or rezoning.
- 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.
- Relative support: yes Family-occupancy ADU explicitly permitted in single-family zones.
Contacts
Staff: Planning & Zoning Division (Zoning Administrator (Title V Reorganized Development Code)), Building Inspections (Engineering Department) (Building Permits & Inspections)
Utilities
- Water: Washington Water Utility · 30d connect · $4,500
- Sewer: Washington Sewer / Wastewater · 30d connect · $5,500
- Electric: Washington Electric Utility · 21d connect · $1,800
- Gas: Washington Gas Utility · 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; HOA covenants restricting ADUs are enforceable.
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) and Building Code (eCode360), adopted 2009-01-01, last amended 2024-04-01
- 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 single-family and two-family 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. - 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. - 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.
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 Codes
- 63068
- 63090
Post Office
- 123 Lafayette St, 63090
- 1777 Washington Xing, 63090
Locale Names
- Cpu Downtown Washington