How many ADUs are allowed per lot in Seattle?

Seattle allows two ADUs per lot citywide in zones where a principal dwelling is allowed (any mix of AADU/DADU). Local overlays (e.g., Shoreline/historic) may add constraints.

What size can my ADU be?

Up to 1,000 gross square feet per ADU in Neighborhood Residential and RSL; city updates align with the state minimum of 1,000 sf across zones where ADUs are allowed.

Do I have to live on the property?

No. Owner-occupancy is not required for the principal home or ADUs.

No. Seattle does not require on-site parking for ADUs. You generally may not remove existing required off-street parking unless it is replaced elsewhere on the lot.

What about height and setbacks?

Height and setbacks vary by zone and roof type. State law requires ADU height parity with the principal dwelling or at least 24 feet, whichever is smaller. Seattle standards meet or exceed this; verify your zone’s numbers.

Are there impact or street improvement fees?

Seattle does not generally impose street improvements for ADUs, and ADUs are exempt from new public street improvements. A separate King County sewer capacity charge applies.

Regulations • Seattle (City)

Seattle ADU Regulations — 2025 City Playbook

Seattle aligns with Washington’s ADU preemption: two ADUs per lot, up to 1,000 sf per unit, no owner-occupancy, and no on-site parking required—plus pre-approved plans to speed permits.

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TL;DR: Seattle allows two ADUs per lot in zones where a principal home is allowed. Up to 1,000 sf per ADU, no owner-occupancy, and no on-site parking required. Typical DADU height and setbacks depend on zone/roof type; state parity rules apply. Confirm your exact zone and any overlays before submitting.

State & city quick nav

Detached accessory dwelling unit behind a primary house in Seattle — Seattle backyard cottage (DADU) example; pre-approved plans can accelerate permitting.

Seattle rules at a glance

City standards (summarized)

How many?
Two ADUs per lot (any mix of AADU/DADU) where a principal dwelling is allowed. {{verify locally}}

Size
Up to 1,000 sf per ADU in NR/RSL; citywide updates align with the 1,000 sf state minimum across allowed zones. {{verify locally}}

Owner-occupancy
Not required for the principal unit or ADUs.

Parking
No on-site parking required for ADUs; do not remove required spaces unless replaced on-site per code.

Height & setbacks
Vary by zone/roof type; parity with principal or at least 24 ft per state rules. Check your zone’s specifics. {{verify locally}}

Fees
City street improvements not required for ADUs; separate King County sewer capacity charge applies.

Zones: Neighborhood Residential (NR1/NR2/NR3), Residential Small Lot (RSL), and Lowrise (LR) allow ADUs with differing dimensional limits. Shoreline/historic overlays may add constraints. {{verify locally}}

Quick visuals

Exact height/setbacks depend on zone, roof form, lot width, and overlays. See SMC and SDCI Tips. {{verify locally}}

How to permit an ADU in Seattle

Check zoning & overlays. Confirm zone (NR/RSL/LR/etc.) and whether Shoreline or historic rules apply.
Choose your path. AADU, DADU, or conversion of an existing structure (e.g., garage/outbuilding).
Pick plans. Use pre-approved DADU plans for speed or submit custom plans.
Prep submittals. Site plan, floor plans, elevations, energy/mechanical/electrical forms. See SDCI Tips 116A/116B.
Apply online. Submit via the Seattle Services Portal; pay fees. Note: a King County sewer capacity charge applies to ADUs.
Build & inspect. Schedule inspections (foundation, framing, rough-in MEP, insulation/energy, final). {{verify locally}}

Seattle does not require public street improvements for ADU permits. Overlays and utilities still apply. {{verify locally}}

What this means on the ground

Two ADUs unlock multigenerational living + rental income on one lot.
No parking requirement simplifies sites near transit and narrow lots.
Pre-approved plans can trim review time and reduce design cost.
Parity standards (height/setbacks/design) reduce ADU-only constraints.

Always confirm zone-specific numbers and any overlays before you draw your final set.

Mini case notes

NR lot near transit: 1,000 sf DADU + internal AADU; no on-site parking needed; height/roof per NR standards. {{verify locally}}

RSL/LR small lot: 1,000 sf cap; watch height/setback specifics and any small-lot rules; pre-approved plan route recommended. {{verify locally}}

Seattle ADU FAQ

Is there a minimum lot size for a DADU?

In Neighborhood Residential zones, a minimum 3,200 sf lot is typical for DADUs. Other zones differ. {{verify locally}}

Can I remove my existing required off-street parking?

Not to build the ADU unless you replace that required space elsewhere on the lot in a permitted location. {{verify locally}}

What’s the fastest permit path?

Use a pre-approved DADU plan (ADUniverse). It’s designed for expedited review via the Seattle Services Portal.

Can I condo (sell) an ADU separately?

State law allows separate sale (e.g., condo/unit lot) where local processes exist; Seattle’s code updates include condo provisions. {{verify locally}}

What inspections are required?

Typically foundation, framing, rough-in MEP, insulation/energy, and final. Your inspector will confirm the sequence. {{verify locally}}

Next steps

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Official resources

SDCI — Accessory Dwelling Units (overview & FAQs)
SDCI Tips — 116A (AADUs) & 116B (DADUs)
ADUniverse — Pre-approved DADU Plans & Homeowner guide
Seattle Services Portal — Apply & manage permits
King County — Sewer capacity charge
Washington — HB 1337 bill summary