Accessory Structures in Pierce County, WA (2026)
5 county-level rules, plus city-specific rules for 2 cities in Pierce County, Washington.
Verified from official government sources
ADU Rules
Unincorporated Pierce County allows accessory dwelling units under PCC 18A.37.120. Two ADUs are permitted per lot inside an Urban Growth Area and one outside, attached or detached, with size caps and a building permit.
Pierce County Accessory Dwelling Unit (ADU) Rules
Few RestrictionsPierce County Code 18A.37.120
Two ADUs are allowed per lot of record as an accessory use within the Urban Growth Area (UGA) and one ADU outside of the UGA in conjunction with any single-family residential structure in all zones where that use is permitted.
ADU Impact Fees
Pierce County caps traffic impact fees on accessory dwelling units at half the rate charged for the primary house. Under PCC 4A.40.030, an ADU built in unincorporated Pierce County cannot be billed more than 50 percent of the traffic impact fee assessed for the principal single-family dwelling on the same lot.
Pierce County ADU Impact Fee Cap
Some RestrictionsPCC 4A.40.030, footnote (3)
(3)Impact fees for the construction of ADUs may not exceed 50 percent of the impact fees that would be charged for the construction of the principal unit.
Shed & Carport Rules
A shed, garage, greenhouse, or storage building in unincorporated Pierce County can go up to 576 square feet without a principal use on the lot, but on residential lots under 1 acre total detached accessory structures cap out at 2,000 square feet. Interior and rear yard placement is separately limited to a share of the yard plus a 3-foot setback.
Pierce County Shed & Accessory Structure Rules
Some RestrictionsPierce County Code 18A.37.020
A. In all zones, there shall be no limit as to the number of accessory uses allowed on a lot provided: 1. The use meets all applicable development regulations; and 2. The use is accessory to a lawfully established residence, except for structures listed in PCC 18J.17.020.E. B. Structures typically accessory to a residence, such as garages or carports, greenhouses and storage buildings, may be p...
Garage Conversions
Converting a garage into living space in unincorporated Pierce County requires a building permit, and creating an independent unit is treated as an accessory dwelling unit under PCC 18A.37.120 with size limits.
Pierce County Garage Conversion Rules
Some RestrictionsPierce County Code 18A.37.120
Square footage for an ADU shall be calculated based on the interior habitable area of a dwelling unit, including basements and attics, but not including a garage or accessory structure.
Tiny Homes
Unincorporated Pierce County treats a tiny home under 400 square feet on a permanent foundation as a residence permitted like a single-family house, while recreational vehicles cannot be used as a permanent dwelling beyond 120 days.
Pierce County Tiny Home Rules
Some RestrictionsPierce County Code 18A.38
Occupancy of a recreational vehicle, travel trailer or tent, or combination thereof, for more than 120 days in any 12-month period shall be considered permanent occupancy.
Accessory Structures by City in Pierce County
2 cities in Pierce County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
All Pierce County ordinances
See every category we cover for Pierce County: parking, noise, fences, fires, animals, pools, and more.
Pierce County Ordinance Hub →