Pierce County, WA ADU (accessory dwelling unit) Rules and Regulations (2026)

Most of Pierce County’s ADU rules depend on whether the lot is inside the Urban Growth Area. Inside it, a lot can carry two ADUs of up to 1,000 square feet each; outside it, one ADU of up to 1,250 square feet. Before either number matters, confirm the parcel is in unincorporated Pierce County and not inside a city.

The county rules below come from Pierce County Code 18A.37.120, last amended by Ordinance 2026-523s, the cottage-housing package the County Council passed on June 23, 2026, which took effect August 1, 2026. For the statewide picture, see our Washington ADU laws guide.

Who reviews your lot

A Gig Harbor mailing address can also cover unincorporated land on the Gig Harbor Peninsula, which follows the county code, not the city’s.

Find the zone and community plan first

Pierce County organizes allowed uses by community plan area, then by urban and rural zones. The county’s zoning help page links the use tables for each plan area: Frederickson, Parkland-Spanaway-Midland, South Hill, and Browns Point/Dash Point have urban tables; Key Peninsula, Alderton-McMillin, Anderson/Ketron Islands, and Upper Nisqually have rural tables; Gig Harbor Peninsula, Graham, and Mid-County have both. Land outside any community plan uses the countywide urban and rural tables. Setbacks sit in Table 18A.15.040-1 (urban) and 18A.15.040-2 (rural).

You need a parcel number or site address to look up zoning. The ADU summary on that same page cites an older code section; the current text of PCC 18A.37.120 controls.

Number and type of ADUs

Under PCC 18A.37.120:

• Two ADUs per lot of record inside the UGA; one outside. They must be accessory to a single-family residential structure in a zone where that use is permitted.
• Any mix of attached and detached.
• ADUs don’t count toward density.
• A detached ADU may be any dwelling permitted in the zone.
• No ADUs accessory to a cottage unit in a cottage-housing development.

Size and how Pierce measures it

An ADU can be no larger than 1,000 square feet inside the UGA and up to 1,250 square feet outside it. Square footage is the interior habitable area of the unit, including basements and attics, but not a garage or other accessory structure.

A model that fits the rural cap can be too big for an urban lot. Compare floor areas on our models page against the right number before you choose a plan.

Setbacks and placement

• New detached ADUs can’t go in the required front, side, or rear yard setback.
• Conversions inside the UGA. An existing detached structure can become an ADU inside the side or rear setback.
• Design rules within 100 feet of the house. When the ADU is less than 100 feet from the principal structure, a detached ADU can be no closer to the front lot line than the front of the house, unless it is a converted existing structure. Waterfront lots under the shoreline code are excepted.
• UGA setback reductions. Within that design subsection, UGA side/interior and rear setbacks for an ADU may be reduced to 5 feet, and the rear setback to 2 feet where the ADU abuts an alley. On corner lots, the principal structure’s front setbacks apply.

Height

A new detached ADU can’t be taller than the principal dwelling’s building height. Two exceptions: an ADU over a detached garage may exceed the house height by one story, and no ADU can be limited below 24 feet unless the house itself is limited below 24 feet. Confirm the zone’s height limit and how the county measures your roof form before finalizing a model.

Parking and street improvements

The county’s parking table lists one space per ADU. PCC 18A.37.120 removes that extra space for properties within one-half mile of a major transit stop, as defined in RCW 36.70A.681. Farther out, each ADU gets parking under the table. Street improvements can’t be a condition of ADU approval.

Septic, wells, and sewer

The Tacoma-Pierce County Health Department regulates septic systems and wells in the county. Its procedures include ADU density guidance and development rules for properties with existing septic systems. If your lot is on septic, ask the Health Department which procedure applies before you choose a bedroom count. On a sewered urban lot, confirm the connection path and capacity with the sewer provider.

Applying

The owner applies for a building permit for the ADU. The application has to show that every size threshold and design standard is met. Plan on a site plan showing setbacks, distance to the house, driveway, parking, and utilities, and a floor plan that proves the square footage.

Pierce County is developing a pre-approved ADU design program. According to the county, pre-approval covers design review only — septic or sewer capacity, setbacks, and critical areas are still reviewed on your site.

Older summaries you may find

Some older summaries of Pierce County’s ADU rules, including the county’s own zoning help page, describe an “owner occupant” applicant, and some describe a notice recorded with the County Auditor before occupancy. Neither appears in the current text of PCC 18A.37.120. Ask Planning and Public Works whether any recording is still part of your permit conditions rather than relying on an older summary.

Rentals and state law

Inside the UGA, state law bars owner-occupancy requirements and caps ADU impact fees at half of the principal unit’s (RCW 36.70A.681). Counties may still restrict ADUs used as short-term rentals (RCW 36.70A.680). Pierce County’s vacation-rental rules in PCC 18A.37.040 apply to accessory dwellings, including an affidavit filed with the county, and the county is reviewing its short-term rental regulations. The ADU building permit doesn’t cover vacation-rental use on its own.

How a factory-built ADU fits

Wolf homes are modular, built to Washington’s residential code — not HUD (the federal standard for manufactured homes) manufactured homes. Washington L&I reviews the factory plans and oversees factory inspections, and the home carries its L&I insignia before shipping. The county still reviews placement, the foundation, utility connections, and site inspections.

Pierce County sites are well beyond the standard 20-mile delivery in Wolf’s published installation scope, so transport and setting are quoted after we see the site. See cost and the Washington ADU cost guide for planning ranges.

What Wolf checks first

1. City or unincorporated — then inside or outside the UGA.
2. The community plan area, zone, and setback table.
3. Distance from the house, which decides whether the 100-foot design rules apply.
4. Septic, well, or sewer capacity.
5. Transit distance and the parking layout.
6. Delivery route, turns, and where a crane or trailer can set the home.

Send us the address and we’ll tell you which rule set applies and what size fits before you pay for design.

This is general information, not legal advice. Recheck the current statute, local code, and permit packet before you file — jurisdictions amend forms and effective dates.