Can I Build an ADU on My Lot?

Most standard single-family lots in the Seattle area can fit a DADU — but whether yours can comes down to a handful of specifics. Here's what actually decides it, and how to know for sure.

The factors that decide it

• Lot size — bigger lots have more room to meet setbacks and coverage limits.
• Setbacks — required distance from property lines shapes where a DADU can sit.
• Lot coverage — the share of your lot that structures can cover, which caps the footprint.
• Height limits — how tall the unit can be, which affects a one- vs. two-story design.
• Trees — protected or protected Tier 2 trees can limit where you build.
• Critical areas — steep slopes, wetlands or flood-prone ground (ECAs) add requirements and cost.

Seattle is more ADU-friendly than it used to be

The rules have loosened. Seattle expanded ADU allowances in 2019, and Washington's statewide reforms pushed that further — many lots can now have up to two accessory units, with owner-occupancy and parking mandates rolled back. Read the details in Washington's ADU laws and the ADU & DADU FAQ.

The January 2026 Neighborhood Residential rewrite went further again, replacing flat unit caps with density based on lot area. On many lots the first question is no longer whether an ADU is allowed, but how many homes the lot supports and whether you want two ADUs or a larger project.

Signs your lot is a good candidate

• A reasonably flat, buildable area — often in the backyard
• Room to meet setbacks around the new structure
• Utilities (sewer, water, power) reachable without extreme runs
• No major critical areas across the build site

The only way to know for sure

A quick look at your lot online only gets you so far — the real answer comes from a feasibility study that checks your zoning, setbacks, trees, critical areas and utilities against what you want to build. That's exactly how we start every project. Trees are the item that most often changes an answer here — how Seattle's tree rules affect a DADU explains which trees on a lot are protected and how much ground they take out. If your lot is inside Seattle, the numbers those standards produce are in how big a DADU can be in Seattle. The formal version of this check is a feasibility study — what it checks and why it comes first. Zoning is only one of the two permissions you need — if your neighborhood has covenants, see whether an HOA can stop an ADU in Washington. Estimate a build with the cost calculator, or get in touch and we'll tell you what your lot can do.

One more gate that only applies to some lots: if the property is on an on-site septic system rather than a public sewer, the county health department answers the capacity question before zoning matters at all. The arithmetic and the paperwork are in building an ADU on a septic system.