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 "What an ADU does to your property tax in King County". 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 "What an ADU does to your property tax in King County" 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: Read: What an ADU does to your property tax in King County (/insights/property-tax-impact-king-county).

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).

March 12, 2025 · 5 min

What an ADU does to your property tax in King County

King County Assessor data, applied to a real Seattle parcel — the assessed-value bump and the tax math.

How the Assessor values an ADU

The King County Assessor reassesses parcels annually and adds the value of new improvements at the next assessment cycle following completion. ADUs are valued using a cost-approach model that considers size, finish quality, and comparable improvements in the area.

The Assessor's eRealProperty portal lets you see the parcel-level valuation history for any address.

Sources:King County Assessor

The tax math

Seattle's combined property tax rate hovers near $9–$10 per $1,000 of assessed value depending on the school district and tax-code area. A DADU that adds $250,000 of assessed value increases annual property tax by roughly $2,250–$2,500.

On a unit renting at $2,650/month gross, that tax increase is offset within the first month of annual rental income.

Senior, disability, and low-income exemptions

Existing King County exemption programs apply to the primary parcel and are not lost when an ADU is added. The added improvement value is taxed at the standard rate; the exempted portion of the underlying property remains exempt.

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 "What an ADU does to your property tax in King County". 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 "What an ADU does to your property tax in King County" 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: Read: What an ADU does to your property tax in King County
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