5 county-level rules, plus city-specific rules for 3 cities in Pierce County, Washington.
Verified from official government sources
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 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.
Unincorporated Pierce County exempts a one-story tool or storage shed of 200 square feet or less from a building permit under PCC 17C.30.040, but larger structures and setbacks under Title 18A still apply.
Pierce County Code 18A.37.020
In residential zones, on lots of less than 1 acre in size, detached unoccupied accessory structures shall not exceed 2,000 total square feet.
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 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.
A building permit is required for all carports in unincorporated Pierce County regardless of size, and carports must meet Title 18A accessory-structure setbacks and the 3-foot minimum setback under PCC 18A.37.020.
Pierce County Code 18A.37.020
Detached, single-story accessory structures, except for accessory dwelling units, may occupy 25 percent of the total area of an interior yard and shall maintain a minimum 3-foot setback.
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 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.
3 cities in Pierce County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
See every category we cover for Pierce County — parking, noise, fences, fires, animals, pools, and more.
Pierce County Ordinance Hub →