An empty corner of the yard is not enough to establish that an accessory dwelling unit will work on your property. The first questions are about the parcel, utilities, and intended use. Answering them before choosing a floor plan can prevent you from designing a building that does not fit the site’s requirements.
An ADU provides independent living space with facilities for sleeping, cooking, and sanitation. It may be within the main home, attached to it, or detached. A detached accessory dwelling unit is often called a DADU. This checklist helps Pierce County homeowners organize an initial feasibility review; it is not an approval to build.
Confirm the jurisdiction and urban growth area status
Find the parcel number and verify whether the property is in unincorporated Pierce County or an incorporated city. Then confirm zoning and whether the parcel is inside an urban growth area, or UGA. These are separate questions: a property can be outside city limits but inside a UGA.
The Pierce County Development Center links property research, zoning and site-constraint tools, and septic records. If your property is within Tacoma or another city, use that city’s ADU requirements rather than transferring the County’s standards to the address.
Check the actual allowance for your parcel
For unincorporated Pierce County, the current County ADU code allows two ADUs per lot inside the UGA and one outside it, in conjunction with a single-family residential structure where that use is permitted. Attached and detached combinations are allowed. ADUs are not permitted as an accessory to a cottage-housing unit.
The same section limits an ADU to 1,000 square feet inside the UGA and allows up to 1,250 square feet outside it. The calculation is based on interior habitable area, including habitable basement and attic space, rather than simply a building’s exterior footprint. A building permit is required.
Those allowances are a starting point, not proof that a particular lot can accommodate the maximum. Washington’s urban ADU framework preserves public-health, building, and environmental requirements and does not override restrictions arising from septic infrastructure, critical areas, or unsuitable physical conditions. Verify the rules for the address before treating a statewide ADU headline as a project approval.
Map the usable space before selecting a plan
Gather a site plan or survey if you have one. Mark the main house, existing structures, access, utility routes, and known easements. Ask which setbacks, height limits, lot-coverage rules, parking provisions, and fire-access requirements apply to the proposal.
Also investigate slopes, drainage, wetlands, shoreline restrictions, and other critical-area constraints. A large lot can still have a small practical building area. Online maps are useful screening tools, but they should not be treated as a substitute for whatever site verification or studies the reviewing authority requires.
Pierce County’s pre-application services guidance identifies Level 1 screening as required for properties with critical areas or shoreline designation and those served by onsite septic systems or private wells. Check that requirement early. The screening report identifies applications, reports, and studies needed for the project; it is not a building permit.
Resolve wastewater and water supply early
For a sewer-served property, ask the sewer provider about connection requirements, capacity, and the proposed route. For a septic property, obtain the system’s record drawing and information about the drainfield and reserve area before placing a building or driveway on the plan.
The Tacoma-Pierce County Health Department’s development guidance evaluates septic capacity, system condition, wastewater flow, reserve area, and water supply. An ADU that increases flow may need a new septic design when the existing system cannot support the proposal. Septic-related minimum lot-area requirements are a separate check; unused bedroom capacity alone is not a complete feasibility answer.
If the home uses an individual well, include water review and required testing in the discussion. Do not assume that a well serving the existing house automatically establishes approval for another dwelling. Also have the appropriate professional evaluate electrical service and utility extensions before treating the building shell as the whole project.
Compare a conversion with a detached build
An existing basement or garage may be worth evaluating, but existing walls do not make it ready for residential use. Ask about moisture, ceiling heights, structural condition, exits, fire separation, insulation, and the routes for plumbing and ventilation. Identify which conditions can be checked now and which require further investigation.
A detached unit offers a different arrangement, with its own building footprint and utility connections. Consider the walking route from parking, privacy between the homes, outdoor space, and access for maintenance. During construction, equipment and material delivery also need a workable route.
Compare these options using the same intended use and level of finish. The smallest building is not necessarily the simplest project if it requires difficult site work or major utility changes.
Separate a promising design from an approved project
As of October 2026, Pierce County describes its pre-approved ADU design program as under development. The County also explains that pre-approved designs do not eliminate site review for matters such as septic or sewer capacity, setbacks, and critical areas. Do not assume that a plan offered elsewhere is pre-approved for your parcel.
Before advancing to detailed design, assemble a short feasibility file:
- Confirmed jurisdiction, zoning, and UGA status.
- A site sketch showing the proposed unit and known constraints.
- Initial answers about sewer or septic, water, and electrical service.
- A list of required reviews, studies, permits, and unresolved questions.
- A preliminary scope covering site work, utilities, construction, and restoration.
Considering a detached ADU? Contact Broussard Construction at (360) 509-0280 or phillip@broussard-homes.com. Share your address, the intended use, and any plans or property records you already have so the conversation starts with the site—not just a floor plan.
Washington contractor registration: BROUSHS858KO.