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 exceptional 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.

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. Estimate a build with the cost calculator, or get in touch and we'll tell you what your lot can do.