Building an Accessory Dwelling Unit in the Greater Seattle Area

An accessory dwelling unit is a second, independent home on a lot that already has a house on it: a small detached backyard cottage (called a DADU, for detached ADU), a converted basement or attic apartment, or an addition with its own entrance, kitchen, and bath. In Seattle, Bellevue, Kirkland, Issaquah, Newcastle, and SeaTac, ADUs have become one of the most common ways homeowners add living space, house family, or create a rental unit without buying a second property.

What changed, and why it's easier now than it used to be

For years, Seattle's ADU rules were restrictive: you needed a parking space for the new unit, the owner had to live on-site, and only one ADU per lot was allowed. Seattle's 2019 ADU ordinance removed the owner-occupancy requirement and the off-street parking mandate citywide, and raised the size limits for both attached and detached units. A statewide law passed in 2023 (HB 1337) pushed the same direction further, requiring cities across Washington to allow at least two ADUs per lot (for example, a basement conversion plus a detached unit) and capping how much cities can restrict them with owner-occupancy rules, parking requirements, or oversized lot minimums.

The practical effect: on most single-family lots in Seattle, and increasingly in the surrounding cities as they update their codes to comply with state law, you can now build one attached unit and one detached unit on the same property. That said, every city in the region administers this differently. Bellevue, Kirkland, Issaquah, Newcastle, and SeaTac each have their own zoning code, lot coverage rules, height limits, and setback requirements, and they're not identical to Seattle's or to each other's. The first real step on any ADU project here isn't design, it's confirming what your specific lot, in your specific city, is zoned to allow.

What drives the cost

There's no single number that applies to every ADU, because the variables are large: whether you're converting existing space or building new, whether the lot needs a new utility connection (water, sewer, gas, electrical service) run out to a detached structure, how the site slopes, and the finish level you choose. A basement conversion that already has structural walls, a foundation, and nearby utility lines is a different project than a detached unit built from scratch on a sloped backyard that needs new utility trenching. Get a firm number for your lot before you plan around one from a neighbor's project or a number you saw online, because site conditions move the budget more than almost anything else.

Permitting takes longer than construction planning assumes

Every jurisdiction in the greater Seattle area reviews ADU permits against its own zoning and building code, and that review is almost always the longest phase of the project, not the shortest. Homeowners often plan a timeline around the number of weeks it will take to build and then discover permitting alone runs months. Design decisions that seem cosmetic, like where the unit's entrance sits relative to the property line, can trigger a full setback review if they weren't checked against the code first. Starting the permit application with a design that's already been checked against your city's specific zoning saves real time, because a design that has to be revised mid-review restarts the clock.

Why Passive House standards make sense on a small building

A detached ADU is a compact, well-defined building envelope, which is exactly the condition where Passive House construction methods pay off fastest: tight air sealing, high-performance windows, and continuous insulation matter more per square foot on a small structure than on a large one, because there's less volume to buffer against outside temperature swings. The result is a unit that costs less to heat and cool over time and stays comfortable without oversized mechanical equipment. It's not the only way to build an ADU well, but on a small footprint it's one of the more direct ways to get a unit that performs the way it looks like it should.

When an ADU isn't the right project

If what you actually want is one more bedroom, a home office, or space for aging parents who don't need a fully separate kitchen and entrance, a full ADU may be more project than the problem calls for. An addition or a remodel of existing space can solve the same need for less disruption and a shorter permitting path. An ADU makes the most sense when you want a genuinely independent unit: separate entrance, separate kitchen, and the ability to rent it, house a family member long-term, or eventually sell the property as a two-unit lot.

Love Construction builds ADUs and DADUs across Seattle, Bellevue, Kirkland, Issaquah, Newcastle, and SeaTac, including Passive House certified units, and works the zoning and permitting question for your specific lot before design starts.