Seattle Garage Conversion Cost 2026: Bonus Room to ADU

As Marcus Reid, Licensed GC and PMP writing for Kolmo Construction, I get one version of this question almost every week: "We have a two-car garage we never park in — what would it cost to turn it into real space?" The range is wide, because a garage conversion in Seattle can be anything from a $10,800 insulated bonus room to a $129,000 permitted ADU. This guide breaks down exactly what each scope costs in 2026, what drives the number up, and where the SDCI permit line gets drawn — so you can place your project on the map before you call anyone.

Quick Summary & Key Takeaways

• A simple kept-door bonus room runs $10,800–$12,800. The garage door stays, the space gets insulated, wired, and finished — legal and useful as a gym or office, but not habitable "living space" by code.
• A conditioned bonus room with the door walled in runs $14,200–$21,300, plus the door infill, which is quoted after a site visit — Seattle's single most common garage conversion, and the point where you gain permanent, assessed square footage.
• Add a full bath and you're at $29,900–$37,000; a full ADU-style suite is $76,500–$98,500 in a 1- or 2-car garage. The ADU path delivers the best return on rentable space in Seattle's tight market.
• Almost every conversion needs an SDCI permit, and homes built before 1978 should carry a +8–10% contingency for asbestos, lead paint, and knob-and-tube surprises.

What a Garage Conversion Costs in Seattle (2026)

Garage conversions don't price by "size" the way a deck or a paint job does — they price by scope, because the fixed costs (walling in the door, one bathroom, one heating zone) dominate. Here are the four scopes we actually build, priced 2026-09-29 (the 1-car kept-door room 2026-10-08) in Kolmo's estimators for a 1- or 2-car garage, with all-in ranges (labor, materials, permit and base contingency included). The ADU-style suite's price includes the garage-door infill; for the room conversions, the infill, cladding match and new windows are quoted after a site visit, as are the kept-door room's insulated door, outlets, lighting and heat:

A finished room in a garage starts at $9,500, the low end of the range the estimator shows and the same starting price as a finished basement room; a smaller job is topped up to it on its own line, which sets the 1-car kept-door room at $10,800. The ADU-style suite follows the ADU minimum the same way: its range never starts under $65,000, which sets the 1-car builder suite at $76,500.

Because lot and garage sizes vary, here's the same money mapped against the three garage footprints we see most often in Seattle:

The scope ranges above match our live Seattle garage conversion cost guide as of 2026-10-08, which we keep updated as material and labor costs move. In the footprint table, each room conversion shows its estimate's own planning range (−12% / +18%) and the ADU-style suite runs from builder to premium finishes. For an instant ballpark on the conditioned-finish portion of the work, the closest interactive tool is our Seattle basement finishing cost calculator, which prices the room tiers above — basement and garage build-outs share roughly 80% of the same scope — then add the garage-specific items below.

Line-Item Cost Breakdown

Where the money actually goes on a typical 2-car (450 sq ft) conversion, from the line items in Kolmo's estimators, priced 2026-09-29 with the base contingency included. Lines marked "allowance" are quoted after a site visit:

• Garage-door-to-wall infill: $3,000–$8,000 (allowance for the room conversions; the ADU-style suite's per-sq-ft rate includes it) — remove the door and tracks, frame a stud wall matching the adjacent exterior, set a window or patio door, match the cladding (the trickiest part on older Ballard and Wallingford homes with one-off siding), and patch drywall. Add $1,500–$3,000 for a dedicated entry door with a covered step.
• Demo & site prep: $1,000–$3,000 (allowance) — strip existing finishes, haul-out, protect the adjacent structure.
• Slab leveling / moisture mitigation: $1,103–$4,746 — a vapor barrier and sealant treatment on the easy end; an interior perimeter drain and sump pump on the hard end. Leveling a badly settled slab is quoted on site, and hillside lots in West Seattle or Beacon Hill may need a French drain first (a separate line).
• Framing & interior structure: $11,529 — partition walls, headers, blocking, plus the drywall, basic wiring and lighting, paint and trim the estimator prices in the same line.
• Ceiling height / truss raise: $0, or $8,000–$18,000 (allowance) — standard 8-ft garages are fine; a 7-ft or open-to-truss garage needs an engineered fix if you want a legal bedroom (7-ft clear required).
• Electrical: $1,418 for a 100A sub-panel if your main is near capacity; basic circuits, outlets and lighting are in the framing line, not itemized.
• Plumbing (bath / kitchenette scopes): $7,886 for a full bath's plumbing, framing, tile and finish labor — rough to the main stack, supply and waste.
• HVAC: $2,520–$4,520 — extend the main-home ducts ($2,520) if your system has spare capacity, or install a ductless mini-split ($4,520) for a dedicated, independently controlled zone.
• Insulation & drywall: $1,264 — wall and ceiling insulation to Washington energy code; drywall, tape, and paint are in the framing line.
• Flooring: $2,599 in waterproof LVP — LVP, engineered wood, or carpet over the prepped slab.
• Egress window (legal bedroom only): ~$2,360 — cutting a code-compliant opening and well per IRC R310. Non-negotiable if anyone will sleep there.
• Bath fixtures & finishes: $5,922 — shower, toilet, vanity, vent fan, tile.
• Kitchenette (ADU scope): $10,595 — sink, fridge, induction cooktop, cabinets.
• Soundproofing to the main home (ADU): $1,500–$4,000 (allowance) — an STC-50+ wall/ceiling assembly so the unit lives independently.
• SDCI permit & fees: $864–$1,762 in the room-conversion estimates (SDCI's 2026 building-permit fee on each job's value), $3,200 in the ADU-style suite — see below.
• Pre-1978 contingency: +8–10% of the project total (8% for a room conversion, 10% for an ADU-style suite) — abatement and rewiring for the things that surface during demo.

Do I Need a Permit to Convert a Garage in Seattle?

Almost always, yes. The moment a garage becomes habitable space, you're adding assessed square footage and changing the building's use, which triggers a Seattle Department of Construction and Inspections (SDCI) construction permit (Addition/Alteration). The only conversion that usually doesn't is the kept-door bonus room, where the space stays non-habitable — though new electrical or a mini-split still needs its own permit.

Here's what changes the answer as scope grows:

• A legal bedroom requires a compliant egress window per IRC R310 and a 7-ft clear ceiling.
• A full ADU must meet energy-code envelope standards, soundproofing, and may get its own street address.
• Seattle now allows up to two ADUs on most single-family lots (for example, one attached AADU plus one detached DADU) under the rules implementing Washington's HB 1337 — which is exactly why garage conversions have surged. The full rule set is in our Seattle ADU cost guide.

On fees: a conversion valued around $84,500 (a 2-car ADU-style suite with standard finishes in our estimator) carries roughly $3,376 in SDCI building, plan-review, technology, and surcharge fees on the full-review path, with 75% due at application acceptance. If you build to a pre-approved DADU plan, most of the plan-review fee is waived. SDCI publishes the current schedule on its ADU permits page. We handle the SDCI submittal and inspections as part of the job — Kolmo Construction (Lic# KOLMOL*753JS) carries the permit so you're not chasing it.

Why a Garage Conversion Pays Off Right Now

• You already own the shell. The slab, three walls, and roof are built — you're paying for finish and systems, not a foundation, which is why conversions beat ground-up ADUs on cost per square foot.
• The two-ADU rule unlocked rental income. A converted garage in Greenwood or Ravenna can rent as a legal unit, and Seattle's constrained market keeps occupancy high.
• No lost yard. Unlike a detached backyard cottage, a conversion adds living space without giving up garden, patio, or P-patch room.
• Aging-in-place and multigenerational living. A ground-floor suite with a no-step entry is the most-requested layout we build for families planning ten years ahead.

HVAC and Moisture: The Two Decisions That Make or Break a Conversion

Two garage-specific choices drive both comfort and budget. HVAC comes down to extending your existing ductwork — cheap and effective if your furnace has 500–800 sq ft of spare capacity, which most Seattle systems do — versus a ductless mini-split that gives the space its own thermostat. Mini-splits are the default for any unit used on a different schedule than the main house, and they're rebate-eligible when paired with envelope upgrades. Moisture is the silent budget-killer: older garage slabs often lack a vapor barrier, so a proper conversion installs one under the finished floor. Skipping it is the single most common reason a converted garage smells musty within a year. Seattle City Light and PSE both offer efficiency incentives worth asking about before you pick equipment.

Attached vs. Detached Garage: Does It Change the Cost?

Yes — and it's the first thing we check on a site visit. An attached garage converts into an attached ADU (AADU): it shares a wall, a roofline, and usually the home's existing water, sewer, and electrical service, which keeps utility costs down and simplifies HVAC tie-in. A detached garage — common on older Beacon Hill and Ravenna lots — becomes a detached ADU (DADU), and that independence cuts both ways. You gain a truly separate unit (better for rental privacy and resale), but you take on running water, sewer, and a dedicated electrical feed across the yard, which can add $8,000–$20,000 in site utilities depending on the trench distance. The flip side: a detached garage often qualifies for Seattle's pre-approved DADU plan set, which waives most of the plan-review fee and shortens the permit timeline. Tell your contractor which one you have before the first number is quoted — it moves the budget by tens of thousands.

Contractor Red Flags to Avoid

• "You don't need a permit for that." On a habitable conversion, you almost certainly do — and unpermitted square footage becomes a problem at resale and with insurance.
• Cash-only or no written contract. A real contractor gives you a line-item scope and a signed agreement.
• No license verification. Confirm any contractor at the Washington State L&I "Verify a Contractor" tool (lni.wa.gov) before you sign — check the bond and active registration.
• A lowball bid with no slab or moisture line. If the quote doesn't mention the floor assembly, they haven't looked at your garage.
• No plan for matching the exterior. Walling in the door without matching siding is the tell of a crew that will leave you with a patch that screams "former garage."

Your Pre-Project Checklist

1. Decide the scope honestly — bonus room, guest suite with bath, or full ADU — because it sets everything downstream.
2. Measure the garage and note the ceiling height (8-ft is ideal; 7-ft limits bedroom options).
3. Check whether your home predates 1978 and budget the +8–10% contingency if so.
4. Confirm your panel capacity — a sub-panel is a common, plannable add.
5. Decide HVAC strategy: duct extension vs. dedicated mini-split.
6. If anyone will sleep in the space, plan for an egress window from day one.
7. Verify your contractor's license and bond at lni.wa.gov.
8. Get a line-item bid that separately calls out infill, slab/moisture, and permits — not a single lump sum.

Ready to start? Run your numbers with our Seattle garage conversion cost guide or the free Seattle home remodeling cost calculator, then contact Kolmo at (206) 410-5100 or visit kolmo.io/contact for a free estimate.

— Marcus Reid, Licensed GC, PMP, Kolmo Construction

Sources and pricing date. Garage figures were priced 2026-09-29 (the 1-car kept-door room, at its job minimum, 2026-10-08) from Kolmo's estimators for the scopes stated above: the room conversions with the basement calculator, the ADU-style suite with the garage-conversion type of the ADU calculator, summarized on the garage conversion cost guide. Lines marked "allowance" are not estimator outputs. How the estimators are built: kolmo.io/how-estimates-work. Seattle permit fee: Kolmo's Seattle permit fee guide (SDCI 2026 Fee Subtitle) and SDCI's ADU permits page. Planning figures, not quotes.