What is a DADU in Seattle?
A DADU — Detached Accessory Dwelling Unit — is a small, fully independent home built on the same lot as a primary single-family house. In Seattle it is regulated under Seattle Municipal Code 23.44.041 and reviewed by the Seattle Department of Construction & Inspections (SDCI). DADUs have their own kitchen, bathroom, and entrance, and can be rented long-term to anyone, including non-family tenants.
Size and height limits
Seattle caps DADU gross floor area at 1,000 sq ft. Up to 250 sq ft of attached garage can be added on top of that. Height limits depend on roof type and how far the DADU sits from the rear lot line:
- Flat roof: 18 ft
- Pitched roof (4:12 or steeper): up to 22 ft
- Pitched roof with 10+ ft rear setback: up to 24 ft — enables a full second story
Lot size, setbacks, and placement
The minimum lot size for a DADU is 3,200 sq ft. The DADU must sit behind the rear façade of the main house, with at least 5 ft from side and rear lot lines and at least 5 ft of separation from the primary structure. Maximum lot coverage including all structures is typically 35% (with some exceptions for small lots).
How many ADUs are allowed?
Seattle allows two ADUs per single-family lot: one attached ADU (AADU) inside or attached to the main house, plus one detached ADU (DADU) in the backyard. Both can be rented separately. There is no owner-occupancy requirement on either unit.
Parking
No off-street parking is required for an ADU in Seattle. This rule applies citywide as of 2019 and is reinforced by Washington HB 1337 (2023), which prohibits cities from requiring parking for ADUs within a half mile of major transit.
Permit process and timeline
There are two paths through SDCI:
- Pre-approved DADU plans — SDCI maintains a library of pre-vetted DADU designs. Permits typically clear in 2–6 weeks.
- Custom design — Architectural and structural review through SDCI standard intake. Plan on 8–14 weeks plus correction cycles.
Construction itself runs 6–8 months for a typical DADU once permits are in hand.
2026 cost in Seattle
A fully turnkey DADU in Seattle in 2026 — design, engineering, permits, construction, utility connections, and certificate of occupancy — runs:
- 400–500 sq ft studio DADU: $185,000–$220,000
- 600–800 sq ft 1-bed DADU: $200,000–$280,000 (most common)
- 900–1,000 sq ft 2-bed DADU: $270,000–$350,000
Frequently asked questions
What is the maximum DADU size allowed in Seattle?
Seattle allows a detached accessory dwelling unit (DADU) up to 1,000 sq ft of gross floor area, plus up to 250 sq ft of attached garage. Height is capped at 18 ft for a flat roof and up to 22–24 ft for pitched roofs depending on roof slope and rear-yard setback. (Source: Seattle Municipal Code 23.44.041.)
Do I need to live on the property to build a DADU in Seattle?
No. Seattle removed the owner-occupancy requirement in 2019. You can build a DADU and rent both the main house and the DADU. Washington State HB 1337 (2023) further preempts owner-occupancy rules statewide.
How many ADUs can I have on one Seattle lot?
Up to two ADUs per single-family lot — one attached ADU (AADU) inside the main house plus one detached ADU (DADU) in the backyard. Both can be rented.
What lot size do I need for a DADU in Seattle?
The minimum lot size for a DADU in a single-family zone is 3,200 sq ft. Lots smaller than 3,200 sq ft can still add an attached ADU but not a detached one.
What are the setback requirements for a Seattle DADU?
5 ft from side and rear lot lines is the standard. The DADU must sit behind the rear façade of the main house, with at least a 5-ft separation between the two structures.
Do I need an off-street parking space for a DADU in Seattle?
No. Seattle eliminated the off-street parking requirement for ADUs citywide in 2019. Lots within a quarter mile of frequent transit had it removed earlier.
How long does it take to get a DADU permit in Seattle?
Using a SDCI pre-approved DADU plan set: 2–6 weeks. A custom design: 8–14 weeks of permit review, plus 2–4 weeks of intake/corrections. We track every milestone in writing.
How much does a DADU cost in Seattle in 2026?
A turnkey 600–800 sq ft DADU in Seattle runs $200,000–$280,000 fixed-price, including design, engineering, permits, construction, and certificate of occupancy. Smaller 400 sq ft units start near $185,000.