Can You Build a House and a DADU on One Lot?
By Eric Dexter, Owner & Builder ·
Here's a move more Seattle homeowners are making: building a new custom home and a detached ADU (DADU) on the same lot, at the same time. Under Washington's current rules many lots allow it — and doing both in one build unlocks real advantages.
Is it allowed?
On most single-family lots in the Seattle area, yes — recent Washington ADU reforms let many lots have a primary home plus one or two accessory units, and owner-occupancy requirements were rolled back. Exact standards vary by city, so your lot's specifics get confirmed in a feasibility study.
Why build them together
- Shared site work — one mobilization, one set of utilities to plan, less duplicated cost
- Rental income from day one — the DADU can help carry the mortgage
- Higher property value — two legal homes on one lot
- Flexibility — family in one, rental in the other, and roles can swap over time
How the numbers work
A combined build costs more up front than a house alone, but the DADU adds an income stream and value that a single home doesn't. See whether an ADU pays off and estimate both with the cost calculator.
One thing to check before settling on a configuration: the house and the DADU both count against the lot's density allowance, and many lots now support more than two buildings. What you can build on a Seattle lot works through the arithmetic.
Build both with one team
Because we self-perform sitework through finishes, building a home and a DADU together is one coordinated project, not two. Learn more about our custom home building, or tell us about your lot.
