There are four ways to add a second unit to a SeaTac property: convert the garage, convert the basement, build an attached addition, or put a detached cottage in the back yard. Which of the four is open to you is decided mostly by the lot, not by preference. Rear yard depth after setbacks, where the sewer lateral runs, and how far the ground falls from the street to the back fence will usually rule out at least one option before anybody draws a floor plan. Reading those conditions early is what keeps a design from being redrawn twice.
The five lot conditions that decide it
What is left of the back yard after setbacks. A detached unit has to sit clear of the property lines and clear of the house. Draw the setbacks on a site plan first and see what rectangle survives. A deep lot with 30 feet behind the house has options. A shallow lot with 12 feet does not, no matter how large the yard looks when you are standing in it.
Lot coverage. The house, the garage, the deck, the patio and the new unit all count toward a maximum footprint for your zone. Plenty of SeaTac lots have the yard space for a cottage and no coverage left to build it with. Removing the existing garage is often what frees up the number.
Where the sewer runs, and which way the ground falls. A detached unit needs a drain line with fall to the main. If the back of the lot sits lower than the connection, the unit needs a sewage ejector pump, which is a pump, a basin, an alarm and a maintenance item for the life of the building. It is not a deal breaker. It is a cost and a design decision that should surface in week one, not during excavation.
Grade. Parts of SeaTac have real slope. A sloped building pad means an engineered foundation, retaining, and sometimes a crane or a pumped pour because a truck cannot reach the back yard. Grade is the single most common reason two identical-looking units on the same street cost very different amounts.
Access and trees. Ask how the excavator, the concrete and the lumber package physically reach the build area. A side yard that narrows to four feet, a fence that has to come down, a neighbor's driveway, a significant tree with a protected root zone: each of these adds either cost or a redesign. Alley access, where you have it, is worth a lot.
SeaTac writes its own zoning code, and its setback, height and lot coverage numbers are not Seattle's. So do Bellevue, Kirkland, Issaquah and Newcastle. What a friend built two miles away in another jurisdiction tells you very little about your own parcel.
What each option is actually good for
Garage conversion. The cheapest path when the slab, the walls and the roof are sound. You are working inside an existing envelope, so there is no new foundation and usually no new roof. The catches are ceiling height, an uninsulated slab that needs a floor build-up, and the fact that you lose the garage. If the garage is detached and you convert it, the finished unit is a DADU.
Basement conversion. Good ceiling height and a dry basement make this the fastest route to rental income. Egress windows, headroom under ducts and beams, and moisture control are what decide whether it works. If the basement is damp, fix the water before anything else; a finished unit built over an unresolved moisture problem is money spent twice.
Attached addition. The right answer when there is no convertible space and the back yard cannot take a freestanding building. It shares a wall and a roofline with the house, which keeps utility runs short, and it needs the same foundation and structural work as any addition.
Detached unit (DADU). The most expensive of the four and the one that rents for the most. Its own foundation, its own roof, its own utility runs, real separation for whoever lives there. On a lot that can hold one, it is usually the option worth pricing even if the first number is uncomfortable.
We publish cost ranges for all five configurations, including a Passive House unit, in our guide to ADU building in SeaTac and South King County. For the difference between the attached and detached categories in code terms, and what changed under Washington's 2023 state law, read our ADU vs DADU guide.
Seattle's Department of Construction and Inspections draws the same line: an attached accessory dwelling unit sits within or attached to the house, and a detached accessory dwelling unit sits separately on the property. Both require a permit, and site, zoning, utility and building-code conditions decide which is feasible. Their accessory dwelling unit guidance is a useful reference for the Seattle side of the line, though your own city's code is what governs your parcel.
The SeaTac question nobody asks until too late
Much of SeaTac sits under an approach or departure path. Aircraft noise does not stop a unit from renting here, because the tenant pool includes flight crews, airport staff and maintenance workers who chose the location deliberately. It does decide how the unit feels to live in, and that is a specification question, not a location question.
Noise gets in through windows and through air gaps, in that order. A budget window package and a leaky envelope will be audible on every approach. Laminated or well-specified glazing, a continuous air seal and mechanical ventilation with heat recovery change the experience substantially, and the ventilation part matters more than people expect: a unit that gets filtered fresh air mechanically never needs a window opened for air, which is also what keeps wildfire smoke out during a bad August week. If you are building anywhere near the flight path, spend the money on the envelope rather than the finishes.
When the answer is "one now, one later"
Washington's 2023 ADU law made two accessory units per lot a baseline in most cities covered by the Growth Management Act, and removed owner-occupancy requirements. On a lot that can hold both, converting the basement now and building the detached cottage in a second phase is a reasonable way to spread the cost and still finish with two income streams. It only works if the first project is designed with the second one in mind, because the coverage math, the panel capacity and the utility routing all get harder if you use them up on phase one without planning ahead.
When an ADU is the wrong project
If the goal is more room for your own household rather than a separate tenancy, an addition is usually cheaper per square foot than a second unit, because it does not need a second kitchen, a second bathroom, a second entrance or a separate utility strategy. Our Seattle home addition cost guide covers that comparison. And if the house itself needs a roof, a panel or a foundation repair, do that first. A new unit in the back yard does not fix anything in the front one.
The next step on any of these is a site visit. We walk the lot, check the setbacks, coverage, grade, sewer and access, and tell you which of the four options your property will actually support before you commit to a design. Call (206) 339-2718 or tell us about the property, and see what we do on ADU and DADU projects across SeaTac and King County.