Turn the garage into rent — the lowest-cost ADU you can build.
You already have the structure. Converting an existing garage into a studio or 1-bedroom ADU is usually the fastest, cheapest legal unit you can add to your King County lot — if the scope is honest about insulation, moisture, and the slab.
Is this you?
You have a detached or attached garage you barely use, and you want a unit that brings in rent or houses family without building from scratch. The big question in your head is the one we hear most: “$80K to convert a detached garage to a DADU… is that reasonable?” The honest answer is that quotes on the same garage run from the low $60Ks to over $200K — and most of that spread is scope, not the building.
This format fits if you want to:
- Add rental income with the smallest build and the shortest timeline.
- House an adult child or parent in a private, separate space close to the main house.
- Keep total cost down by reusing the existing slab, walls, and roof where they pass code.
What’s included — and what’s not
A garage is a shed until it’s insulated, sealed, and connected. Here’s the line between a real conversion and a cheap quote that skips the parts you can’t see.
| Included | Not included |
|---|---|
| Framing, insulation, drywall to current energy code | The land — you already own it |
| Slab insulation + vapor barrier (the PNW moisture fix) | Major utility upgrades beyond a standard connection |
| Kitchen, bathroom, egress windows, HVAC | A new foundation if the slab can’t be reused |
| Electrical panel work + standard utility connection | Landscaping beyond restoring what we disturb |
| Permit package and inspections | Furniture and appliances above the standard set |
Published price range
All-in means the full project — vertical build, soft costs, sitework, permits, and a standard utility connection. Not the land. These are 2025–26 King County planning ranges at a standard finish; a premium finish moves the per-square-foot number up.
| Size | Vertical build | All-in (standard) |
|---|---|---|
| 350 sf studio | ~$65K–$110K | $90K–$155K |
| 450 sf studio / 1BR | ~$85K–$140K | $120K–$200K |
| 600 sf 1BR | ~$110K–$190K | $155K–$265K |
This is a planning range from real project data, not a bid. Your number depends on site conditions, finish level, and design. We confirm an exact, fixed-scope price on a real quote.
How it works
- Feasibility — we confirm the garage can be converted and what it’ll take (slab, setbacks, utilities).
- Design + permit package — drawings to code, submitted to SDCI or your city / King County DPER.
- Permit review — typically 3–5 months for a garage conversion.
- Build — insulation, moisture control, kitchen, bath, electrical, finishes.
- Inspections + utility connection, then you have a legal, rentable unit.
City notes
Under HB 1337, owner-occupancy and parking requirements are gone across King County, which removes two old obstacles to converting a garage. A previously unpermitted garage conversion is common here — King County’s own permit analysis flagged it — and it can often be brought to legal status as part of the project. In unincorporated King County the path runs through DPER rather than Seattle’s SDCI, and on-septic lots need a capacity check before you start.
It can be, for a small, simple conversion on a sound slab. But quotes on the same garage commonly range from the low $60Ks to over $200K. The difference is almost always scope — whether the bid includes slab insulation, vapor barrier, egress, a full kitchen and bath, and the utility connection. A typical fully-finished King County garage conversion lands around $120K–$200K all-in, with small studios starting near $90K and larger 1-bedrooms running higher.
