ADU Planning in Seattle

What Seattle Allows

Seattle permits both attached and detached accessory dwelling units on most single-family lots. The city has steadily relaxed ADU rules, removing the parking requirement and the owner-occupancy mandate in most cases, which has made ADUs viable for far more homeowners than a decade ago.

Still, the rules are specific: maximum size limits, height limits for detached units, setback requirements, and coverage calculations all apply. Reviewing the city's current ADU checklist before any design work is one of the highest-leverage steps you can take.

Lot Coverage and Setbacks

Detached ADUs need to fit within your lot's remaining coverage allowance and respect required setbacks from side and rear property lines. In practice this means the buildable footprint is often smaller than the maximum allowed square footage — so site planning comes first, then floor plan.

Utility Connections and Access

An ADU usually needs its own utility connections or capped shared capacity. Water, sewer, electrical, and sometimes gas all factor in. Some lots can't add a second sewer lateral without significant work, which can shift the budget materially. Confirm utility feasibility early — it's a common ADU deal-breaker.

Parking Is Generally No Longer Required

Seattle removed the off-street parking requirement for ADUs in most zones. This is what makes many back-lot ADUs feasible at all. If you want to include a parking space anyway, plan it explicitly — it changes the site plan and the cost.

Design for Renter Durability or Family Comfort

If you're building a rental, choose finishes and systems that survive tenant turnover. If you're building for family, design for aging-in-place and long-term comfort. These goals aren't the same, and the finish selections, layout, and even mechanical systems should reflect which one you're optimizing for.