“Garage Conversion ADU Seattle Eastside: Cost, Permits & Design

*By David Meade, AIA, NCARB | Piper Cole Architects*

> TL;DR: Converting an attached or detached garage into a legal ADU on the Seattle Eastside costs $80,000–$200,000 — roughly $200,000–$450,000 less than building a new detached cottage. Washington HB 1337 (effective July 2025) removed the biggest legal blockers, including the non-conforming setback rule that killed most Bellevue and Kirkland projects before. What still catches homeowners off guard: Washington Energy Code compliance, slab moisture mitigation, and ceiling height viability.

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Does a Garage Conversion Qualify as an ADU — and Does the Square Footage Count?

Yes, a converted garage qualifies as an ADU in every Eastside city we work in. The question homeowners get wrong is whether that garage square footage eats into the ADU size cap.

In Kirkland, Bellevue, Redmond, and Sammamish, garage area is excluded from the ADU size calculation when measuring converted living space. A 480 sq ft detached garage converts to a 480 sq ft ADU — and that entire 480 sq ft is available to you without touching the 1,000–1,200 sq ft maximum.

This matters because most homeowners assume the opposite and underestimate what they can build.

One more thing HB 1337 changed: existing structures — including garages built under older, more permissive setback rules — may be converted to ADUs even if they don’t meet current setback requirements. If your detached garage sits 2 feet from the rear property line, that is no longer a disqualifying condition under state law. We’ve used this provision in recent Kirkland projects to move conversions forward that would have been dead on arrival two years ago.

See our Washington ADU laws 2026 Eastside guide for the full city-by-city regulatory breakdown.

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Attached vs. Detached Garage Conversion: How the Rules and Costs Differ

The attached-versus-detached split is the first design decision that shapes your entire budget.

Attached garage advantages:

• Plumbing and electrical can tie into the main house through the shared wall — saving $15,000–$30,000 in utility rough-ins
• One building permit covers both structures in most Eastside jurisdictions
• Heating and cooling can sometimes be extended from the main system (though a dedicated mini-split is still preferred for tenant independence)

Detached garage challenges:

• New underground utility runs — water supply, sewer lateral, electrical conduit — typically add $20,000–$45,000 depending on distance and site conditions
• Detached units qualify for Seattle’s pre-approved DADU plan program, which can shorten permit review to 2–6 weeks and waive plan review fees

In our Bellevue and Kirkland projects, attached garage conversions consistently come in at the lower end of the $80K–$140K range. Detached conversions, once you price in the utility extension, routinely reach $130K–$200K.

If you’re weighing a garage conversion against a new backyard cottage, visit our ADU vs. DADU King County 2026 comparison.

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Kirkland, Bellevue, Redmond, Sammamish: ADU Rules Side by Side

No two Eastside cities have identical ADU codes. Here’s where things stand in mid-2026:

City	ADUs Allowed	Max ADU Size	Owner Occupancy	Parking	Key Note
Kirkland	2 per lot	1,200 sq ft	Not required	1 space (may waive near transit)	Planning Director may allow larger single-story AADU
Bellevue	2 per lot	1,200 sq ft	Not required (updated 2025)	Waived within 0.5 mi of transit	Previously banned DADUs; HB 1337 changed that June 24, 2025
Redmond	1 AADU or 1 DADU	1,500 sq ft (AADU); 1,000 sq ft (DADU)	Required	1 additional space	Neighbor notification required
Sammamish	1 AADU or 1 DADU	1,000 sq ft DADU	Required	Waived if 4+ spaces on parcel	50% of primary for interior AADUs
Issaquah	1 AADU or 1 DADU	1,000 sq ft (garage/storage excluded)	Required	1 additional (tandem OK)	—

The Bellevue change deserves emphasis. Before June 2025, Bellevue capped AADUs at 800 sq ft, required owner-occupancy, and prohibited detached ADUs entirely. That code is now superseded. If someone told you a garage conversion wouldn’t fly in Bellevue, that advice is outdated.

Our ADU architect Kirkland and ADU architect Sammamish pages cover the permit intake process in each jurisdiction.

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What Washington’s Energy Code Actually Requires When You Convert a Garage

This is where budget surprises live. A garage is an unconditioned space — no insulation, no vapor barrier, typically an unheated concrete slab. When you convert it to habitable living space, the entire envelope must meet current WSEC minimums:

• Walls: R-21 minimum. Standard 2×4 framing requires high-density spray foam or furring out to 2×6 depth to hit this.
• Ceiling/roof: R-49. Blown-in insulation is the most common solution in low-headroom garages.
• Slab/floor: R-10 perimeter insulation under slab. If the slab isn’t replaced, a raised subfloor framing system with R-30 batt is the alternative.
• Energy credits: 1.5 credits for conversions under 500 sq ft; 3.0 credits for units under 1,500 sq ft.
• Vapor barrier: Required under any finished floor. This is the single most common omission in low-bid contractor quotes — and the leading cause of chronic moisture and mold problems within 12–18 months in Seattle’s climate.
• Windows: Must meet current U-factor and SHGC values. Bedroom egress windows require a 5.7 sq ft minimum opening, 24-inch height, 20-inch width, and sill no higher than 44 inches from the floor.

We review energy compliance on every garage conversion project we take on. It is not optional, and contractors who price below $80K for a full legal ADU conversion are almost always underpricing the energy scope.

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The Slab Problem: Floor Options, Ceiling Height, and What Makes or Breaks a Conversion

Two structural questions determine whether a garage conversion is viable and what it will cost:

The floor:

Most Eastside garages have a 4-inch slab-on-grade with no insulation and no vapor barrier. You have three options:

1. Pour new elevated slab with rigid insulation below — $6,000–$12,000. Clean thermal break, full WSEC compliance, loses ~3 inches of ceiling height.
2. Raised subfloor framing system over existing slab — $4,000–$8,000. WSEC-compliant with R-30 batt but loses 8–10 inches of ceiling height. This is often the project killer.
3. Insulated concrete form (ICF) panel system — mid-range cost, minimal height loss, growing in use on Eastside projects.

The ceiling:

The minimum ceiling height for a habitable room in Washington is 7 feet. Most modern Eastside garages have 8-foot walls and clear this threshold. Older garages — particularly pre-1980 detached garages — often have 7-foot walls. Once you account for a raised subfloor framing system, you’re at 6 feet 2 inches. That unit cannot be a legal bedroom.

Raising a truss system to gain headroom costs $8,000–$18,000 with structural engineering. In our experience, this expense rarely pencils out unless the garage is large (500+ sq ft) and the conversion is being positioned as a rental unit.

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Garage Conversion ADU Cost on the Eastside: What $80K–$200K Gets You

The cost range is wide because the scope varies dramatically. Here’s how the line items break down for a full legal ADU conversion:

Line Item	Cost Range
Garage door infill (siding match)	$3,000–$8,000
Slab moisture mitigation + leveling	$1,500–$6,000
Electrical panel upgrade + wiring	$2,500–$7,000
HVAC (mini-split preferred)	$3,500–$6,500
Insulation + drywall	$3,000–$8,000
Egress window (installed)	~$7,000
Bathroom rough-in + fixtures	$3,500–$9,000
Permit fees (Kirkland/Bellevue)	$2,400–$18,000
Structural engineering (if required)	$2,000–$4,000
Sales tax (King County: 10.25%)	On all materials/labor
Pre-1978 contingency (asbestos/lead)	+12% on total budget

Permit timelines:

• Seattle SDCI: 8–14 weeks standard; 2–6 weeks with pre-approved DADU plans
• Bellevue: approximately 60 days
• Kirkland / Redmond: 3–8 weeks

From first architect meeting to certificate of occupancy, plan for 9–18 months.

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Garage Conversion vs. Building a New DADU: The Cost-Benefit Case

The financial comparison is straightforward:

	Garage Conversion ADU	New Detached ADU (Ground-Up)
Cost range	$80,000–$200,000	$280,000–$650,000
Cost per sq ft	$200–$350	$400–$650
Timeline	9–14 months	12–24 months
Design flexibility	Low–moderate	High
Rental income potential	$1,500–$2,500/month	$2,000–$3,500/month
Property value increase	$200,000–$300,000	$250,000–$400,000
Break-even	6–10 years	9–15 years

The case for conversion is cost and speed. The case for new construction is design quality, rental premium, and long-term property value. We typically see homeowners choose conversion when the garage structure is sound, ceiling height clears 8 feet, and the goal is a near-term in-law suite or modest rental — not a premium two-story rental unit.

For a full side-by-side analysis, see our ADU vs. DADU King County 2026 guide.

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The Permit Process in Kirkland and Bellevue: What Triggers a Full Review

Kirkland (Development Services Center):

1. Pre-application meeting recommended for detached conversions
2. Submit building permit application with architectural drawings, site plan, energy code worksheet
3. DADU plan pre-approval available — shortens review if using a compliant plan set
4. Inspections: framing, insulation, rough mechanical/electrical, final
5. Certificate of Occupancy issued after final inspection

Bellevue (Development Services Department):

1. Intake through Bellevue’s permitting portal
2. ~60-day review cycle post-HB 1337 adoption (June 2025)
3. Parking waiver documentation required if within 0.5 miles of transit
4. Energy code compliance worksheet must accompany structural drawings

What triggers a full structural review: Any modification to roof framing, truss system, or load-bearing walls — including a ceiling raise — requires a licensed engineer’s stamp. In our Kirkland and Bellevue projects, we include structural drawings in the base scope whenever ceiling height is marginal or the garage predates 1990.

HB 1337 setback protection: Submit documentation showing the existing structure predates current setback requirements. The city cannot require relocation or demolition as a condition of the ADU permit.

See our home remodel architect Kirkland page for more on how we manage the permit process alongside design.

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Frequently Asked Questions

Q: Does my garage square footage count toward the ADU size limit?

A: No. In Kirkland, Bellevue, Redmond, and Sammamish, existing garage area is excluded from the ADU size calculation. A 500 sq ft garage converts to a 500 sq ft ADU without touching the city’s size cap.

Q: Can I convert my detached garage if it’s too close to the property line?

A: Yes, under Washington HB 1337 (effective July 1, 2025), existing structures may be converted to ADUs even if they don’t meet current setback requirements. The non-conforming setback is no longer a disqualifying condition for conversion projects.

Q: What’s the minimum ceiling height for a legal ADU bedroom in Washington?

A: Seven feet. If your garage has 7-foot walls and you install a raised subfloor framing system (which loses 8–10 inches), you will not clear the habitability threshold. Either a new elevated slab or a truss raise is required.

Q: Do I have to replace the parking space I’m converting?

A: HB 1337 removes the parking replacement requirement for properties near transit. In Kirkland and Redmond, one additional off-street space may still be required for properties not near a transit stop. Bellevue waives the requirement within 0.5 miles of a transit stop.

Q: What does a vapor barrier have to do with my garage conversion budget?

A: Everything. Washington’s wet climate creates hydrostatic pressure under concrete slabs. Without a vapor barrier beneath the finished floor, moisture migrates upward, saturating insulation and framing within 12–18 months. Contractors who omit this to hit a lower bid price are setting up a mold remediation project. We specify vapor barrier on every conversion — it’s a required item, not an upgrade.

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Ready to find out if your garage qualifies? David Meade, AIA, NCARB offers a free ADU feasibility consultation for Kirkland, Bellevue, Redmond, and Sammamish homeowners. We review your garage dimensions, ceiling height, setback situation, and city code — and tell you straight whether a conversion pencils out before you spend a dollar on drawings.

Schedule your free ADU feasibility consultation →

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Sources

1. City of Kirkland Development Services Center — Garage Conversions. kirklandwa.gov
2. HouseHack Seattle — Washington HB 1337 ADU Zoning Overhaul. househackseattle.com
3. TrueTown — Bellevue ADU Construction Law Changes 2025. truetown.com
4. MyBuildingPermit.com — Construction Tip Sheet 25: Converting a Garage to Living Space. mybuildingpermit.com
5. DwellingIndex — Washington ADU Laws 2026: Rules by City. dwellingindex.com