ADU Costs in Bellevue, Washington

Planning-level pricing guide by ADUWizard.com
Updated for 2026 budgeting

Bellevue, Redmond, and Kirkland are the top end of Washington’s ADU market. Costs track Seattle closely and often edge a little higher, mostly because Eastside homeowners tend to build to a higher finish level. Two things work in your favor here: plan review is frequently faster than Seattle’s, and Bellevue allows a noticeably larger ADU.

This is the Eastside version of the real budgeting guide, written for a Bellevue DADU, garage conversion, or basement unit in 2026.

Quick disclaimer: A budgeting guide, not a quote or legal advice. Eastside bids move with slope, soils, tree rules, and finish level, and Washington sales tax (around 10.3% here) rides on top. Use these ranges to compare bids.

Bellevue ADU Cost in 2026 (Quick Answer)

Project scenario	Typical size	Bellevue / Eastside all-in
Garage conversion	400–650 sf	$95k–$230k
Basement / interior ADU	450–800 sf	$180k–$340k
Attached ADU / addition	500–900 sf	$240k–$420k
Detached new-build (DADU)	500–1,200 sf	$320k–$620k+
Prefab / modular installed	500–900 sf	$270k–$500k

All-in cost per square foot on the Eastside runs roughly $300–$500 for conversions and $380–$650 for detached units.

Bellevue ADU Rules That Affect Cost (2026)

Bellevue’s ADU reform brought the city in line with, and in places beyond, Washington’s statewide HB 1337 framework.

Rule	Bellevue standard (2026)	Why it matters for cost
ADUs per lot	Up to two in most residential zones	Plan a detached plus a conversion
Max detached size	Up to 1,200 sf floor area	The largest by-right ADU of the WA metros we cover
Max height	24 ft detached (28 ft over an existing accessory structure)	Supports a real two-story unit
Setbacks	Per middle-housing dimensional standards, varies by zone	Confirm your zone before you design
Parking / owner-occupancy	Eased near transit; no owner-occupancy	Removes financing and site-cost drags

What drives cost on the Eastside: higher finish expectations, larger lots with longer utility runs, tree retention, and critical-area and slope rules in the hillier neighborhoods. The bigger 1,200 sf allowance is great, but square footage is exactly what raises the total.

Bellevue ADU Cost by Type and Size (2026)

Type	Typical size	All-in range
Garage conversion	400–650 sf	$95k–$230k
Basement / interior	450–800 sf	$180k–$340k
Attached addition	500–900 sf	$240k–$420k
Detached DADU	500–1,200 sf	$320k–$620k+

Size	Conversion	Detached DADU
500 sf	$110k–$240k	$320k–$500k
800 sf	$180k–$340k	$380k–$580k
1,200 sf (Bellevue max)	n/a	$460k–$620k+

Example Bellevue Budget: 800 sf Detached DADU (Mid-to-High Finish)

Category	Budget
Design + engineering	$22,000–$45,000
Permits + fees	$12,000–$30,000
Sitework + utilities	$30,000–$80,000
Foundation + envelope	$130,000–$220,000
MEP	$60,000–$105,000
Finishes (Eastside standard)	$70,000–$140,000
Sales tax + contingency	$55,000–$100,000
Total all-in	$380,000–$620,000+

Bellevue ADU Permit Timeline

Plan review in Bellevue, Redmond, and Kirkland typically runs 3–8 weeks, faster than Seattle’s SDCI. Total project time still lands around 9–14 months once design and construction are included.

FAQs (Bellevue / Eastside)

How much does an ADU cost in Bellevue in 2026?

Budget roughly $95k–$230k for a garage conversion and $320k–$620k+ for a detached DADU. Eastside projects tend to build to a higher finish level, which pushes totals toward the top of those ranges.

Is it more expensive to build an ADU in Bellevue than Seattle?

Usually a little. Base construction is similar, but Eastside projects tend to build to a higher finish level and sit on larger lots with longer utility runs. Plan review, though, is often quicker.

How big can an ADU be in Bellevue?

Detached ADUs are allowed up to 1,200 sf of floor area, larger than Seattle, Tacoma, or Spokane, with a 24-foot height limit (28 feet when built over an existing accessory structure).

How long does plan review take on the Eastside?

Roughly 3–8 weeks in Bellevue, Redmond, and Kirkland, versus 8–16 weeks in Seattle. Complex sites and design review extend it.

What is the cheapest ADU option in Bellevue?

A garage conversion, generally $95k–$230k all-in when the structure and utilities cooperate.

For the statewide view, see our Washington ADU cost guide, compare Seattle ADU costs, and check the Data Hub for current ranges. City permitting details are at the City of Bellevue ADU reform page.