Building a DADU or ADU in Seattle

What you can build

• Number: up to two ADUs per lot, attached, detached or stacked, so two DADUs are allowed (SMC 23.42.022).

• Size: 1,000 sq ft of gross floor area with up to two bedrooms; 1,200 sq ft with three or more. Up to 250 sq ft of attached garage and underground stories do not count.

• Height: the zone's limit applies. In Neighborhood Residential zones that is 32 ft.

• Lot coverage: 50 percent maximum in Neighborhood Residential zones. Since January 21, 2026, an ADU follows the same development standards as the main house (Council Bill 120993).

• Parking: none required for an ADU.

• Owner occupancy: not required. State law (RCW 36.70A.681) bars cities from requiring it.

• Selling: an ADU can be sold as a condominium unit.

What can slow it down

• Trees. In Neighborhood Residential zones a Tier 2 tree can come out only where keeping it blocks the allowed lot coverage, a unit, access or utilities. Removed trees must be replaced.

• Environmentally critical areas (ECA). Steep slopes, landslide areas and other ECAs add review and rule out the pre-approved plan path.

• Utilities. Plan for a separate electrical permit, a plumbing permit and often a side sewer permit. Every ADU needs the King County sewer capacity charge form.

The permit steps

1. File a Building & Land Use Pre-Application on the Seattle Services Portal.

2. Schedule intake. SDCI has noted intake waits of 2 to 4 months.

3. Upload the site plan, floor plans of the house and the ADU, elevations, energy calculations and structural drawings where needed.

4. Pay the intake and plan review fees, answer corrections, then schedule inspections.

A DADU is reviewed without public comment or appeal.

The pre-approved plan path

Seattle publishes pre-approved DADU designs. They apply to NR1, NR2 and NR3 lots with no ECA and under 750 sq ft of ground disturbance. The City reviews only the site-specific parts: zoning, foundation, utilities and drainage.

Review times

SDCI goals as of October 1, 2025, in calendar days of City review time at the 75th percentile. SDCI says total time including the applicant's turns runs roughly twice these figures.

• Pre-approved DADU plans: goal 30 days, current 60 days.

• Middle housing, including single-family homes: goal 60 days, current 117 days.

• Single-family addition or alteration: goal 30 days, current 64 days.

Common questions

Can I build two backyard cottages?

Yes. Up to two ADUs are allowed per lot, and both can be detached.

Do I have to live on the property?

No. Owner occupancy cannot be required, and SDCI has a form to release old owner-occupancy covenants.

Do I need a parking space?

No. Seattle requires no off-street parking for an ADU.

Is there a faster path?

Pre-approved DADU plans, on eligible NR lots, have the shortest SDCI review goal: 30 City days.

How SASHKOV handles it

• Checks the lot first: zone, lot size, critical areas, trees, utilities and access, in writing.

• Checks whether a pre-approved plan fits the lot, or prepares a site-specific permit set.

• Brings in the surveyor, structural engineer or other licensed professional the project needs, under one contract.

• Prepares the permit set, answers corrections and coordinates inspections through issuance.

Where a building requires a licensed architect or engineer of record, SASHKOV brings them into the contract.

More on SDCI: the City of Seattle breakdown.