Seattle Attached Accessory Dwelling Unit Permits Since 1990

Permit records from the City of Seattle for building permits that create or demolish attached accessory dwelling units (AADUs). Records span from 1990 through the current year quarter and include permits that are completed or currently issued. The data tracks the number of units added or demolished per housing type, with a single permit potentially appearing multiple times for mixed-unit projects.

Use Cases

• Analyze trends in AADU creation versus demolition over time using permit completion dates.
• Study the evolution of regulated housing unit types, such as the introduction of small efficiency dwelling units in 2016.
• Examine permit density and project scale by aggregating unit counts per address where multiple permits may exist.
• Model the impact of building code changes on the prevalence of specific unit types visible in the data over different time periods.

Strengths

• Temporal coverage spans over three decades, from 1990 through the current year quarter.
• Data is sourced from the authoritative City of Seattle permitting system, providing official records.
• Includes granular detail on unit types as regulated by building codes at the time of permit issuance.

Limitations

• Does not include permits that were issued but not completed, limiting comparability with permit issuance statistics.
• Housing unit type classifications are not standardized and change with building code revisions, complicating longitudinal analysis.
• Sample size and row count are unknown, making it difficult to assess the dataset's statistical power.

Provenance

Source

City of Seattle permitting system.

Collection Method

Extracted permit records for building permits related to attached accessory dwelling units.

Time Range

1990 through the current year quarter.

Freshness

Updated through the current year quarter, with a last metadata update in March 2026.

Geography

City of Seattle, Washington, United States.