ADUs & DADUs in West Seattle: What Homeowners Need to Know Right Now

This is one of the most common questions I'm getting from West Seattle homeowners right now. If you own a home in West Seattle, Alki, Admiral, Beach Drive, North Delridge, or anywhere in between, you've probably heard the term "DADU" more this year than ever before. Backyard cottages are popping up on blocks all over 98116, and for good reason: Seattle's accessory dwelling unit (ADU) rules are the most permissive they've ever been, and the timing lines up well with two things a lot of West Seattle owners are thinking about, rental income and resale value.

Here's a practical, no-fluff breakdown of what's changed, what it means for your lot, and how to start the process without getting lost in SDCI paperwork.

First, the basics: ADU vs. DADU

Seattle recognizes two types of accessory dwelling units:

• AADU (Attached ADU): A unit built into or attached to your existing home, think a basement apartment, a converted garage, or an addition above a garage.
• DADU (Detached ADU): A fully separate, standalone structure on the same lot, the classic "backyard cottage."

Under current rules, most single-family lots in Seattle can have both an AADU and a DADU, up to two accessory units total, in addition to the primary home.

Why this is such a hot topic in 2026

A few things have converged to make this the moment for West Seattle homeowners to pay attention:

The rules got a lot friendlier, and they've now settled. Washington's HB 1337 (2023) and HB 1110 codified statewide what Seattle had already been moving toward: no owner-occupancy requirement, no additional off-street parking requirement, and up to two ADUs per lot. Seattle folded all of this into a single, unified code section (SMC 23.42.022) in 2025, and the permanent middle-housing zoning update took effect in January 2026. After several years of legal back-and-forth over the city's original ADU environmental review, the framework is now considered settled and is being actively enforced, which means less ambiguity for anyone applying today.

Pre-approved plans have shortened the timeline. Seattle's "ADUniverse" library of pre-approved DADU designs can move through permit review in a matter of weeks rather than months, compared to a fully custom design. For homeowners who don't need a one-of-a-kind structure, this is a meaningful shortcut.

The economics make sense here. West Seattle's rental demand, combined with build costs that are still lower than fully custom in-city construction, has made DADUs an increasingly common way for owners to generate rental income, house family members, or simply add usable, code-legal square footage that shows up as real value at resale.

What West Seattle lots specifically need to watch for

West Seattle has a few quirks that make it worth doing your homework before you fall in love with a floor plan:

• Environmentally Critical Areas (ECAs). A meaningful number of West Seattle parcels, especially anything near the bluffs along Beach Drive, Alki, or the ridge above the water, sit near or on steep-slope ECAs. This can affect where a DADU can be sited, what kind of geotechnical review is required, and sometimes whether a detached structure is feasible at all. A stamped topographic survey is usually the fastest way to get a real answer, rather than relying on GIS overlays alone.
• Lot size and coverage. Most lots need to be roughly 3,200 sq ft or larger to accommodate an ADU project, and any new structure counts against your lot's overall coverage limit (commonly 35% in single-family zones). On some of West Seattle's narrower, older lots, this is the real constraint, not the zoning code itself.
• Setbacks and height. A DADU generally needs to sit at least 5 feet from rear and side property lines, with additional setback required as height increases. Height maxes out around 24 feet for a pitched roof.
• Utilities. A detached unit almost always needs its own utility connections or sub-metering, and any new roof or hardscape area can trigger stormwater review, something to flag early if your lot already has drainage quirks (common on the steeper West Seattle streets).

None of these are dealbreakers, they're just the reason a five-minute conversation with SDCI or a quick pre-application review can save you months of redesign later.

The permit process, in plain English

1. Confirm your zoning and lot eligibility. SDCI's online permit and parcel tools will tell you your zoning designation, lot size, and any overlays (ECA, shoreline, flood zone) tied to your specific address.
2. Decide: custom design or pre-approved plan. A pre-approved DADU plan from the ADUniverse library moves through review faster. A custom design gives you more flexibility but takes longer.
3. Consider a pre-application conference. For anything beyond a simple, pre-approved layout, a short meeting with SDCI staff before you formally submit can surface problems (ECA, setback, utility) while they're still cheap to fix.
4. Submit for a building permit. Every ADU project, attached or detached, requires a full building permit, including site plan, structural drawings, and energy code compliance. There's no separate Master Use Permit required for most ADU projects.
5. Plan for review and construction timelines. Realistically, budget 2–6 months for planning and permitting on a DADU, and additional months for construction depending on complexity, contractor availability, and whether utility or drainage work is involved.

Should you actually build one?

This is the question that matters more than the code itself. A DADU or AADU makes the most sense when:

• You want rental income and are comfortable being a landlord (no owner-occupancy requirement means you can rent out either unit, or even the main house, without living on-site).
• You're planning ahead for aging parents, adult kids, or multigenerational living.
• You're preparing to sell in the next few years and want to add legitimate, code-compliant square footage rather than a home addition that eats into your existing living space.

It's worth noting Seattle law is clear that an ADU can't be sold separately, it stays tied to the primary residence, and the whole property transfers as one parcel. So the value is in rental income and resale price, not in creating a separate saleable unit.

Where to start

If you're a West Seattle homeowner weighing whether your lot can support an ADU or DADU or you're trying to figure out how much value one would actually add if you sold in the next year or two, that's exactly the kind of question worth a real conversation rather than a Google search. Every lot in this neighborhood has its own combination of slope, size, and zoning quirks, and the right first step is usually a quick lot assessment before you spend money on architectural drawings.

This post is for general information and reflects Seattle's ADU/DADU rules as of 2026. Requirements can vary by parcel ,always confirm current zoning, setbacks, and permit requirements for your specific address with the Seattle Department of Construction and Inspections (SDCI) before finalizing any plans.

RaeAnne Marcum | REAL BROKER | Team Lead | Seattle | Bellevue | Kirkland | 509.521.5323 | raeannemarcum.com