The most common DADU question we get is 'how big can I build?' — and the honest answer is that no single square-footage cap decides it. Your buildable envelope comes from a stack of rules applied to your specific lot. Here's how those rules interact, and why a feasibility check is worth doing before you hire a designer.
Seattle is now strongly pro-DADU
Since the city's 2019 reforms and Washington's 2023 statewide ADU law, the rules favor building: owner-occupancy is no longer required, parking minimums have largely been removed, and most single-family lots can carry both an attached and a detached unit.
- No owner-occupancy requirement
- Parking minimums removed in most of the city
- Up to two ADUs on most standard lots
What actually decides your size
Four limits set your envelope, and the tightest one wins. We model all four against your parcel so the design is built to what's permittable, not bounced back for redesign.
- Maximum height and roof-form rules
- Rear and side setbacks, plus separation from the main house
- Lot coverage limits across all structures
- Floor-area ratio (how much building the lot allows)
The pre-approved plan shortcut
Seattle maintains a library of pre-approved DADU plan sets that skip much of the structural and architectural review. When a standard plan fits your lot, it's the single biggest lever on both cost and timeline — and most expediters never bring it up. We'll tell you honestly whether one fits before you commit to custom design.
Common questions
Do I need a separate address and utility meter?
A permitted DADU is a separate legal dwelling and is typically separately addressed; utility metering depends on your setup. We coordinate the side-sewer and utility details as part of the permit.
Can I rent it out?
Yes. With owner-occupancy gone, a permitted DADU can be rented long-term. We make sure it's a legal, separately-addressed unit so financing and insurance hold up.