Washington County · 32,709 residents
Washington ADU rules
Washington: internal ADUs allowed; detached ADUs allowed with conditions (lot 11,000+ sq ft).
Internal ADU (inside the home)
Detached ADU (backyard building)
Not yet fully verified · last reviewed September 7, 2026 See sources
Washington ADU rules at a glance
| Rule | Internal ADU (basement, attic, inside the home) | Detached ADU (backyard building) |
|---|---|---|
| Allowed? | Allowed | Allowed with conditions |
| Minimum lot | Statewide statute includes a small-lot pathway around 6,000 sq ft for many internal ADUs — confirm how Washington applies it on your zone. | 11,000 sq ft. Statewide detached floor for specified municipalities: roughly 11,000 sq ft on qualifying single-family lots (operative Oct 1, 2026). Local code may allow detached ADUs on smaller lots sooner. |
State law: detached ADUs from October 1, 2026
Washington is a “specified municipality” under Utah S.B. 284. Beginning October 1, 2026, it must permit one detached ADU as a permitted use on a lot of 11,000 sq ft or more that has a detached single-family home. The city can still apply setbacks, height, design, owner-occupancy and limited parking rules, and it may allow detached ADUs on smaller lots. How the state ADU laws work.
How our property map screens Washington lots
The map colors parcels using the rules on this page and the recorded lot size. A color is a planning screen, not a permit; Washington makes the decision.
- "Likely" above 15,000 sq ft is this site's planning band for a large recorded lot. It is not a sentence in the Washington code. Owner occupancy, parking, setbacks, zones, and HOA rules below still have to be checked.
Planned communities, PRDs and PUDs
Reviewed on September 22, 2026 against this city's research file, its published ADU page where one was found, and Utah meeting packets. No adopted sentence was confirmed that allows or bans an ADU in every PUD, PRD, PRUD, or planned community. If the assessor class or subdivision name says PUD, PRD, PRUD, planned unit development, planned residential development, planned community, resort community, or master planned community, the map keeps that lot highly unlikely regardless of size. A plat, CC&Rs, or development agreement can still prohibit an ADU where the zone allows one.
Planning notes for Washington
Washington City directs homeowners to establish ADU qualification before preparing the building-permit application. The published process then uses City Inspect and identifies the builder and relevant project contacts. We can help check the current zoning pathway first and organize the plans and responsible parties for a complete submission.
Bring these to your review
- Parcel zoning and proposed ADU type
- Site plan and design scope
- Builder or owner-builder details
The question to resolve first
Does this parcel qualify under the current ordinance, and which documents belong in the building-permit package?
Sources
We link only to government sources and public meeting records. Codes change; confirm the current version with Washington before you apply.
- Washington City residential ADU permit requirements
Published city submittal guidance
- Utah S.B. 284 (2026), enrolled bill
Statewide detached-ADU requirements for specified municipalities, operative October 1, 2026.
- Official Washington website
Common Washington ADU questions
Can you build an ADU in Washington?
Washington: internal ADUs allowed; detached ADUs allowed with conditions (lot 11,000+ sq ft). This is planning guidance, not a permit. Confirm the zone on your parcel with Washington.
What lot size does Washington require for an ADU?
Internal ADU: Statewide statute includes a small-lot pathway around 6,000 sq ft for many internal ADUs — confirm how Washington applies it on your zone. Detached ADU: 11,000 sq ft minimum. Statewide detached floor for specified municipalities: roughly 11,000 sq ft on qualifying single-family lots (operative Oct 1, 2026). Local code may allow detached ADUs on smaller lots sooner.
What does it cost to build an ADU in Washington?
Wasatch Front planning bands are often about $55,000–$145,000 for an internal suite, $70,000–$175,000 for a garage conversion, and $95,000–$250,000 or more for a new detached unit, before unusual site work. These are planning ranges, not bids.
Can an HOA block an ADU in Washington?
City approval does not override CC&Rs or plat notes. Read the HOA documents and your subdivision plat before you design. A private restriction can stop a unit the city would otherwise allow.
Before you design: general checks for any Utah ADU
General planning advice, not Washington code.
- Confirm the zoning district on the city zoning map. The county assessor’s “residential” class is not zoning.
- Read your HOA CC&Rs and subdivision plat notes; private restrictions can block a unit the city allows.
- Sketch existing and proposed parking. Driveway geometry and access often decide what fits.
- Ask early about sewer, water and electrical capacity; utilities can force a redesign.
- Expanding a building’s footprint usually triggers full setback and height rules.
Nearby city ADU rules
Want to know what fits on your Washington lot?
Free, no obligation. We review your parcel against these rules and, if you want, introduce independent design and construction partners. Partners may pay us for referrals.
Utah ADU Builder provides educational planning guidance. This page is not legal advice and not a permit. Population: UGRC POPLASTESTIMATE / Census-derived municipal estimates (manual fill where UGRC blank). Editorial standards.