What's Required in Washington
- Full architectural and structural plan sets.
- Zoning compliance review (must meet lot occupancy and height limits).
- Separate utility connections or approved shared capacity.
- Must have a permanent kitchen and bathroom.
- Must meet fire separation requirements from the main dwelling.
- Homeowner must reside in either the primary home or the ADU.
How Much Does an ADU Permit Cost in Washington?
Budget $1,500–$5,000 for the Washington permit itself (valuation-based), issued by the Department of Buildings — but treat that as the entry fee, not the total. ADUs are full dwelling units, so the permit stack usually includes building, electrical, plumbing, and mechanical trade permits, plus plan review. One local note: high complexity project requiring full architectural review and multiple trade fees. Get an itemized fee estimate from the department before finalizing your budget; the spread between a simple conversion and a new detached build is wide.
ADU Size Limits and Zoning Rules in Washington
Before drawing plans, check the two gates that sink most ADU projects: size and zoning. Size ceilings vary by lot and ADU type (attached, detached, or conversion), so confirm your specific configuration. Other recorded conditions: zoning compliance review (must meet lot occupancy and height limits); homeowner must reside in either the primary home or the ADU. Setbacks, height limits, and utility connections layer on top of those rules. The Department of Buildings can confirm what your specific lot allows — worth doing before you pay for architectural drawings rather than after.
How to Get a ADU Permit in Washington
Step 1: Zoning Verification
Verify if your property's zone (e.g., R-1, R-2) allows for an ADU.
Step 2: Design & Civil
Develop full architectural plans and a site plan for DC Water and DOB review.
Step 3: Building Permit Application
Submit a 'New Construction' or 'Major Alteration' permit application.
Step 4: Construction Inspections
Series of inspections: Footings, Framing, Trades, and Insulation.
Step 5: Certificate of Occupancy
Upon completion, you must apply for a C of O to legally rent the unit.