Permit Required? Required
Typical Fee Range $1,500–$5,000 High complexity project requiring full architectural review and multiple trade fees.
Fee Method Valuation-based
Apply Online Apply at Department of Buildings →

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 to Get a ADU Permit in Washington

1

Step 1: Zoning Verification

Verify if your property's zone (e.g., R-1, R-2) allows for an ADU.

2

Step 2: Design & Civil

Develop full architectural plans and a site plan for DC Water and DOB review.

3

Step 3: Building Permit Application

Submit a 'New Construction' or 'Major Alteration' permit application.

4

Step 4: Construction Inspections

Series of inspections: Footings, Framing, Trades, and Insulation.

5

Step 5: Certificate of Occupancy

Upon completion, you must apply for a C of O to legally rent the unit.

Frequently Asked Questions

Yes, 'Carriage House' conversions are a common form of ADU in DC, provided zoning allows.
Yes, ADUs typically cannot exceed 35% of the gross floor area of the main house or 450-600 sq ft depending on the zone.
Yes, DOB will assign a 'Unit B' or 'Rear' address for emergency services and mailing.