Washington law describes two different property-tax paths that can involve an ADU. One applies to certain improvements to a single-family dwelling, including an attached or detached ADU. The other allows local governments in specified counties to adopt a separate exemption for an ADU rented to a low-income household. Neither statute gives a parcel-specific tax result by itself.
Improvement exemption after construction
Under RCW 84.36.400(1), qualifying improvements are exempt from property tax for the three assessment years after completion to the extent the improvement value is no more than 30% of the original structure’s value. The law expressly includes an attached or detached ADU. The taxpayer must file notice before construction using the required Department of Revenue or assessor process, and the exemption may not be claimed more than once in five years.
The exemption concerns the qualifying improvement value, not the entire property. Ask the county assessor how it measures the original structure and improvement, which notice form applies, and whether the parcel satisfies the statutory conditions.
Optional local exemption for affordable ADU rentals
The law separately authorizes qualifying local jurisdictions to exempt the value of some ADUs rented to low-income households. A local legislative authority must adopt the option, and county population rules affect which authorities may participate and whether detached units are required. The statute includes a tenant-income ceiling of 60% of adjusted county median household income, rent no higher than 30% of tenant income, a lease of at least 12 months, annual application, and other restrictions.
The local option is not automatic statewide. A county assessor must confirm whether the jurisdiction has opted in, which local rules apply, what evidence is required, and whether the ADU and tenant qualify. The statute says the tax preferences apply to taxes levied for collection in 2026 and after and expire January 1, 2034; check the current law and forms before filing.
These provisions concern property-tax assessment, not land-use permission, permit approval, rental legality, or utility capacity. For other public ADU assistance types, see the grants and programs guide and the guide to funding conditions.