Can I build an ADU or backyard cottage on a lot in Pierce County?

First question: who governs the lot?

The mailing address won’t tell you. A house with a Gig Harbor address may sit outside the city limits, under county rules. Much of South Hill carries a Puyallup mailing address but is unincorporated county. Some addresses that say Tacoma on the envelope are in Parkland, which is unincorporated county. Each of those answers leads to a different code book.

The fastest check is Pierce County’s online parcel map, which shows city limits, zoning and the urban growth area boundary for any parcel. The Assessor-Treasurer’s record for the parcel lists its tax code area as well. Once you know who governs the lot, you know where to call.

What Washington’s 2023 ADU law changed

House Bill 1337 passed in 2023 and set a floor that cities and counties planning under the Growth Management Act have to meet inside urban growth areas. In general terms, it requires allowing at least two ADUs on a lot where a single house is allowed, attached, detached or one of each. It generally bars requiring the owner to live on the property, limits how much parking a city can demand, and restricts impact fees. Local codes were updated on a schedule tied to each jurisdiction’s comprehensive plan, and some places already allowed more than the law required.

Tacoma went further on its own. Through its Home in Tacoma work, the city rewrote residential zoning to allow more kinds of housing in neighborhoods that used to be single-house only. On some Tacoma lots, the backyard cottage conversation can turn into a bigger one.

What the state law didn’t erase is the rest of the code. Height limits, setbacks from property lines, maximum size, lot coverage, tree protection and stormwater rules all still apply, and they vary from one city to the next. A cottage that fits easily behind a craftsman in one town can run into a setback or size cap a few miles away. Codes also keep changing, so read the current version, never a summary from a few years back.

Inside the urban growth area or out in rural Pierce County

The two-ADU rule is written for urban growth areas. A large share of the county lies outside them, including much of the Graham area and the country south toward Eatonville, most of the Key Peninsula, and the rural edges of the plateau. There, Pierce County’s own code decides, and rural rules are generally more limited on how many units a lot can hold and how large they can be.

Rural parcels also bring constraints that have nothing to do with ADU rules. Wetlands and streams come with buffers. Steep slopes may need a geotechnical report. Down in the Puyallup and Carbon river valleys, flood maps and the Mount Rainier lahar hazard zone affect what can be built and how. None of this rules out a second dwelling, but each one can shrink the buildable part of a lot that looks roomy from the road.

Sewer, septic and the limits a lot sets on its own

On a sewered city lot, the question is the connection. Some jurisdictions let an ADU share the main house’s side sewer, and others want a separate one. The sewer utility will have connection charges of its own. On an older Tacoma house, the condition of the existing side sewer matters before you add anything to it, which is one more reason for a sewer scope during inspection.

On septic, the system often decides the whole thing. Septic systems are designed around a number of bedrooms, and adding a dwelling usually means proving the system can handle it or building a larger one, with room on the lot for the drainfield and a reserve area. The Tacoma-Pierce County Health Department oversees septic systems here, and its records for a property, including the original design, are the place to start. On a private well, ask the health department about water supply for a second home as well.

Then there’s the lot itself. An alley behind the house, common in Tacoma’s older grid, makes a detached cottage much easier to build and reach. Big trees and utility easements can both push back. In the newer subdivisions on the plateau and South Hill, homeowners’ association covenants may restrict outbuildings or rentals. Those are private rules, separate from the city’s, so read them and have an attorney look if they seem to conflict with what the city allows.

Buying a house that already has an ADU or a converted garage

A finished cottage out back can be the reason you want a house. Before you pay for it, find out whether it was permitted and given final approval. Start with the city’s or county’s permit records for the address, then the seller’s disclosure statement, which asks about additions and conversions and whether permits were obtained. The assessor’s record of improvements is worth holding up against what you actually see on the lot.

A garage turned into living space without permits is a common find. Sometimes it can be legalized after the fact, and sometimes that takes real construction or can’t be done at all. It also affects financing: the appraiser may give an unpermitted unit little or no value, and your lender decides whether rental income from it can count toward qualifying. Your insurance agent needs to know about it too.

If someone lives in the unit now, you are buying into a landlord and tenant relationship with rules of its own, and Tacoma has local rental rules on top of state law. Short-term rentals carry separate licensing rules in some cities. An attorney can tell you what you inherit at closing.

What to check before you write the offer

• The jurisdiction, zoning and urban growth area status from Pierce County’s parcel map.
• Sewer or septic, and for septic, the health department’s file on the system.
• Permit history for any second unit or converted space already there.
• Homeowners’ association covenants, if there is an association.
• A conversation with the permit counter. Many offer a pre-application meeting for exactly this.

A feasibility contingency in your purchase agreement can give you days to do this homework after mutual acceptance. I talk through how inspection and feasibility periods fit into an offer on the buying page. If the plan is a cottage for a parent and you still own your current home, buying your next home before selling this one covers the order of operations.