Why does this insight matter for WA ADU owners?

Each insight on the Golden State journal targets a specific decision point in the ADU lifecycle — financing structure, design tradeoffs, code changes, market data, or operating decisions for a rental unit. We publish only when we have new primary data from our own bid archive, permit logs, or comp pulls related to "Seattle's ADU code, decoded". The goal is decision-grade information, not generic marketing copy. Go deeper: Glossary: ADU (Accessory Dwelling Unit) (/glossary/adu).

How current is the data in this article?

Insights are dated and the underlying datasets refresh on a rolling basis: cost-per-sqft benchmarks update quarterly from our active Puget Sound bid book, permit timelines update monthly from AHJ logs, rent comps update quarterly from on-market and recently-leased pulls in King/Pierce/Snohomish. Each chart or table notes its as-of date. If you need a custom analysis against your specific submarket, request a feasibility study. Go deeper: Glossary: RRIO (Rental Registration & Inspection Ordinance) (/glossary/rrio).

Can I reuse this analysis for my own planning?

Yes — every insight is written to be actionable. The math is shown, the assumptions are named, and the conclusion is tied to a specific decision (which loan, which finish tier, which AHJ, which size). Feel free to share with your CPA, lender, or family decision-makers. If you'd like a 30-minute walkthrough of how the article's framework applies to your specific lot, book a free scoping call. Go deeper: Glossary: ECA (Environmentally Critical Area) (/glossary/eca).

Does "Seattle's ADU code, decoded" apply to my Puget Sound city?

Most insights are written to apply across the Puget Sound region with the city-specific variables (fees, permit medians, rent comps) called out in tables. When the analysis is city-specific (e.g., Seattle SDCI process), it's labeled in the headline. Use the city pages linked from the article to map the framework to your specific AHJ. Go deeper: Seattle ADU overview (/seattle-adu).

Where do I go from here?

Three good next stops: (1) the ROI calculator if you're evaluating whether the math works on your lot; (2) the permit timeline page for current AHJ medians in your city; (3) the contact form to book a free 20-minute scoping call. Every insight cross-links the most relevant next pages at the bottom. Go deeper: Glossary: ECA (Environmentally Critical Area) (/glossary/eca).

April 8, 2025 · 10 min

Seattle's ADU code, decoded

SDCI Tip 116 plus SMC 23.44.041 in one read — height, setbacks, FAR, and the rules most homeowners get wrong.

Two ADUs per lot, with conditions

Since the 2019 Mandatory Housing Affordability ADU reforms, Seattle allows up to two ADUs per single-family lot — one attached (AADU) and one detached (DADU), or in some configurations two AADUs. Maximum DADU floor area is 1,000 square feet of conditioned space, exclusive of garage and storage up to 250 square feet.

SDCI Tip 116 is the canonical homeowner reference for what is and isn't allowed under the current code.

Sources:Seattle Dept. of Construction & Inspections

Height, setbacks, and lot coverage

DADU height limits in most NR (Neighborhood Residential) zones are 18 feet to the roof peak with a 5-foot bonus for green roofs or sloped roofs above 6:12 pitch. Side and rear setbacks are 5 feet for one-story portions; second-story walls within 10 feet of a side or rear lot line trigger additional setback.

Total lot coverage (house plus DADU plus accessory structures) is typically capped at 35–40% depending on lot size — confirm against your specific NR1/NR2/NR3 designation in SMC 23.44.041.

Sources:Seattle Municipal Code

What homeowners get wrong

Tree protection. Any tree six inches DBH or greater triggers SDCI tree review and can constrain footprint. Stormwater. Lots over 2,000 sq ft of new + replaced impervious surface trigger flow-control requirements. Sewer capacity. Older laterals frequently need replacement, which is the owner's responsibility, not the City's.

None of these block the project — they just need to be designed in, not designed around.

Sources:Seattle Dept. of Construction & Inspections

Owner-occupancy is gone

Seattle removed the owner-occupancy requirement for ADUs in 2019. You can rent both the primary house and the ADU to different tenants and live somewhere else entirely. This is the policy change that made ADU-as-investment underwriting possible at scale.

FROM_THE_FIELD / Built work

Full archive →

FAQ

Frequently asked

Why does this insight matter for WA ADU owners?
Each insight on the Golden State journal targets a specific decision point in the ADU lifecycle — financing structure, design tradeoffs, code changes, market data, or operating decisions for a rental unit. We publish only when we have new primary data from our own bid archive, permit logs, or comp pulls related to "Seattle's ADU code, decoded". The goal is decision-grade information, not generic marketing copy.
Go deeper: Glossary: ADU (Accessory Dwelling Unit)
How current is the data in this article?
Insights are dated and the underlying datasets refresh on a rolling basis: cost-per-sqft benchmarks update quarterly from our active Puget Sound bid book, permit timelines update monthly from AHJ logs, rent comps update quarterly from on-market and recently-leased pulls in King/Pierce/Snohomish. Each chart or table notes its as-of date. If you need a custom analysis against your specific submarket, request a feasibility study.
Go deeper: Glossary: RRIO (Rental Registration & Inspection Ordinance)
Can I reuse this analysis for my own planning?
Yes — every insight is written to be actionable. The math is shown, the assumptions are named, and the conclusion is tied to a specific decision (which loan, which finish tier, which AHJ, which size). Feel free to share with your CPA, lender, or family decision-makers. If you'd like a 30-minute walkthrough of how the article's framework applies to your specific lot, book a free scoping call.
Go deeper: Glossary: ECA (Environmentally Critical Area)
Does "Seattle's ADU code, decoded" apply to my Puget Sound city?
Most insights are written to apply across the Puget Sound region with the city-specific variables (fees, permit medians, rent comps) called out in tables. When the analysis is city-specific (e.g., Seattle SDCI process), it's labeled in the headline. Use the city pages linked from the article to map the framework to your specific AHJ.
Go deeper: Seattle ADU overview
Where do I go from here?
Three good next stops: (1) the ROI calculator if you're evaluating whether the math works on your lot; (2) the permit timeline page for current AHJ medians in your city; (3) the contact form to book a free 20-minute scoping call. Every insight cross-links the most relevant next pages at the bottom.
Go deeper: Glossary: ECA (Environmentally Critical Area)

INTERNAL_LINKS / Deep Map