Building Your Accessory Dwelling Unit in Tacoma Under Tacoma’s Own Rules.

Two accessory dwelling units are permitted on a legally established lot, attached or detached, and the habitable area of each is capped at 1,000 square feet excluding garages and other non-living space. You do not have to live on the lot to build one. This came in with Tacoma’s Home in Tacoma zoning, which took effect on 1 February 2025 under Washington’s HB 1337.

Two caveats worth knowing. Owner-occupancy does apply if you intend to operate either unit as a short-term rental. And the 1,000 square feet is a limit on the unit, not a guarantee about your lot — the ADU still has to meet the lot size, density, height and setback standards of its zoning district. The City’s planning department is the place to confirm how that lands on your specific address.

No. The City states it directly: a pre-approved plan is not pre-permitted. The four base models are already reviewed against building, residential and energy codes, which is what streamlines the permitting, but a permit application is still required and the project still goes through site-specific review.

That review covers the things a plan cannot know about in advance — utility service, critical areas, historic districts, proximity to fire hydrants. Those plans are the City’s, not ours. We have no relationship with the City, and we never draw plans, submit applications or file anything on your behalf.

It depends on the size of the unit, the condition of your site and the finishes you choose, and we will not put a number on it before we have stood on the lot. What we can tell you is where the money tends to go on a Tacoma detached unit.

A detached ADU needs its own water, sewer and electrical service, and on an older property that can mean upgrading the main panel or extending a line further than expected. Permit fees, impact fees and system development charges sit on top of construction cost. Modifications to a pre-approved plan also carry separate designer fees payable to the designer, not to us. Budget a contingency — older lots turn up surprises once the ground is open.

Two phases, and the first is not in anyone’s hands but the City’s. Permitting depends on the department’s current workload and on how complicated your site-specific review turns out to be. Using a pre-approved plan shortens the review of the building itself; it does not remove this phase.

Construction then runs several months depending on the size of the unit and what the site demands. Weather, material availability and the timing of utility inspections all move the end date. We will give you a schedule based on the conditions we actually find, and we will tell you when something changes it.

If you use one of the City’s pre-approved detached models, the building itself is already drawn. You will still need a site plan showing how the structure sits on your property, which usually needs professional drafting.

If you want something the pre-approved models do not cover, you will need to engage an architect or a designer directly. We do not employ architects or designers and we do not draw plans ourselves, so your drawings need to be finished before we start building. Significant structural or floor plan changes can also take a project out of the City’s pre-approved programme altogether, which is worth checking before you commission anything.

Most residential lots in Tacoma can take one, but confirm your own zoning rather than assuming. The City’s online mapping tools will show your zoning district, and the planning department is the definitive source on what your lot can carry.

Detached units go in the rear yard, at least 5 feet from a side property line — 8 feet where a main entrance faces that side — and 15 feet from the rear line, except that no setback is required from an alley. At least 10% of the lot has to stay usable open space. Lots with critical areas or their buffers are treated differently again. If your house sits in one of the nine historic districts, the historic standards take priority where they conflict with the building design standards.

A detached ADU needs its own water, sewer and electrical connections, with external sewer and water service connections required outside the unit. Depending on what is already there, your main service panel may need upgrading to carry the extra load.

The question that usually decides the cost is whether you are connecting to existing lines or need a new extension. We build those connections as part of the site work and to the standards the City and the utility providers set. Talk to the utility companies early — what they tell you about your street can change where the unit is best placed.

Ask what they know about the rules that apply to your lot specifically, and listen for the caveats. A builder who tells you a pre-approved plan is pre-permitted, or that they can move your application along at the City, is telling you something that is not true.

Ask for their Washington L&I registration before you sign anything, and check it yourself. Ask them to walk you through the whole scope — site preparation, foundation, shell, utilities, finishes — and to say plainly which parts they do not do. We do not draw plans, we do not employ architects or engineers, and we do not deal with the City on your behalf. Any builder should be able to tell you where their work stops.