Everything homeowners need to know about building an accessory dwelling unit in Bellevue - size limits, setbacks, permits, and the 2025 reform that finally allowed detached backyard cottages on the Eastside.
Last verified: August 2026
7/10 · ADU-friendly
1,200 sq ft*
Max ADU size
24 ft*
Max height
Not required
Owner-occupancy
By-right
Permit type
This page summarizes Bellevue Land Use Code 20.20.120 (Accessory Dwelling Units) for general reference - it is not legal advice. Regulations change; always confirm current requirements with Bellevue Development Services before designing or applying for permits. See sources below.
Zoning & size requirements
What you're allowed to build
ADU type
Max size
Max height
Notes
Detached (DADU)
1,200 sq ft
24 ft (28 ft if built above an existing accessory structure)
Legal citywide only since July 1, 2025; minimum 4 ft setback from rear and side property lines
Attached (basement, above-garage, or addition)
1,200 sq ft, or 40% of the primary structure's floor area if that's larger
Same as primary structure
Larger primary homes can unlock an attached ADU above the standard 1,200 sq ft cap
Allowed zoning districts
ADUs are allowed on any lot in a land use district that permits single-family dwellings, citywide, under Bellevue Land Use Code 20.20.120. Confirm your parcel's exact land use district and current standards with Development Services.
Additional standards
Setbacks, parking & other rules
Setbacks
Minimum 4 ft from rear and side property lines for a detached ADU; attached ADUs follow the primary structure's setbacks
Parking
None required for ADUs under 1,000 sq ft or within a half-mile of frequent transit; 1 space for ADUs 1,000-2,000 sq ft; 1-2 spaces for ADUs over 2,000 sq ft depending on lot size
Units per lot
Up to 2 ADUs permitted on any lot in a single-family-allowing land use district, in any combination of attached and detached
Owner-occupancy
Not required. Bellevue eliminated its owner-occupancy requirement under Ordinance 6746 in July 2023
Building code
Must meet Washington State Residential Code dwelling-unit standards
Rental rules
ADUs may only be rented long-term (30+ days); whole-unit short-term/Airbnb-style stays are not allowed. No separate ADU registration or public noticing is required as of the 2023 reform
Ordinance history
How Bellevue's ADU rules got here
2021Three-year wait period removed. Ordinance 6567, adopted February 2021, eliminated the requirement that a new single-family home stand for three years before an ADU could be added.
2021Occupancy limits removed. Ordinance 6616, adopted November 2021, eliminated unrelated-occupant caps for residential uses citywide, including ADUs.
2023ADU reform: owner-occupancy eliminated. Ordinance 6746, adopted July 2023, removed the owner-occupancy requirement and dropped separate ADU registration and noticing requirements. Attached ADU size rose to 1,200 sq ft (or 40% of the primary structure if larger). Detached ADUs remained prohibited citywide at this point.
2025Detached ADUs finally allowed. Ordinances 6851 and 6852, adopted June 2025 and effective July 1, 2025, brought Bellevue into compliance with statewide middle-housing law HB 1110 and ADU law HB 1337 - for the first time, homeowners could build a detached backyard cottage, with up to two ADUs allowed per qualifying lot.
Bellevue was one of the more restrictive Eastside cities on ADUs for years, only permitting detached units starting in mid-2025. That's arrived alongside some of the highest housing costs in the Seattle metro, driven in large part by the tech economy anchored by Microsoft's headquarters and a growing Amazon presence nearby - a combination that's made ADUs an increasingly attractive way to add rental income or house family members without leaving the neighborhood.
Permit process
How Bellevue ADU permitting works
1
Confirm your land use district
Verify your lot is in a district that allows single-family dwellings, and check current setback and height standards with Development Services.
2
Decide attached vs. detached
Since the July 2025 reform, you can build one attached and one detached ADU (or two detached units), each up to 1,200 sq ft. Your choice affects setbacks and parking.
3
Prepare your plan set
Site plan, floor plans, elevations, and structural drawings. Use Bellevue's online Permit Fee Estimator to budget before you apply.
4
Submit through MyBuildingPermit.com
Bellevue requires a standard building permit and full development-application review for every ADU, including detached units, garage conversions, and junior ADUs.
5
Plan review & permit issuance
No separate ADU registration or public noticing is required as of the 2023 reform, which simplified this step considerably.
6
Construction, inspections & rental plans
Build and schedule standard inspections. If you'll rent the unit, plan for long-term occupancy only - Bellevue doesn't allow whole-unit short-term rental of ADUs.
Fees and timelines vary by project scope and site conditions - use the city's Permit Fee Estimator and confirm current requirements with Development Services before applying.
Budgeting
What it costs to build an ADU in Bellevue
Garage conversion
Roughly $70,000-$180,000, the least expensive path since the shell already exists
Basement conversion
Roughly $100,000-$220,000, depending on existing ceiling height and utility access
New detached ADU (DADU)
Roughly $150,000-$330,000+, with Bellevue projects commonly landing in the upper half of that range given Eastside land and construction costs
Permit & plan review fees
Vary by project scope; use the city's online Permit Fee Estimator for a project-specific figure before budgeting
Impact fees
Capped by state law at 50% of the principal dwelling's impact fees
These are general ranges from local contractor estimates and regional reporting, not quotes - Bellevue's tech-driven economy and high land values tend to push actual project costs toward the top of these ranges, sometimes above them for larger or more finished units. Confirm a project-specific estimate with a licensed local contractor before budgeting.
Before you apply
Tips for a smooth approval
Confirm detached ADUs are allowed on your lot. Since DADUs only became legal in July 2025, double-check your land use district with Development Services rather than assuming what a contractor or neighbor tells you.
Use the Permit Fee Estimator before you budget. Bellevue's full development-review process makes fees harder to predict up front than in cities with simpler by-right review.
Plan for long-term rental only. Bellevue prohibits whole-unit short-term or Airbnb-style stays in ADUs, so don't design around short-term rental income.
Check the 40%-of-structure rule if your home is large. A larger primary home may unlock an attached ADU bigger than the standard 1,200 sq ft cap.
Take advantage of the removed registration step. Bellevue no longer requires separate ADU registration or public noticing, which used to add time to projects before the 2023 reform.
FAQ
Common questions
Do I need to live on the property to build an ADU in Bellevue?
No. Bellevue eliminated its owner-occupancy requirement under Ordinance 6746 in July 2023, and Washington's 2023 statewide ADU law (HB 1337) bars cities from reinstating it.
Can I build a detached ADU in Bellevue?
Yes, but only since July 1, 2025. Ordinances 6851 and 6852 finally allowed detached ADUs citywide on lots in single-family-allowing land use districts, bringing Bellevue into compliance with state law.
How many ADUs can I build on one lot?
Up to two, in any combination of attached and detached, on a lot in a land use district that allows single-family dwellings.
Do I need extra parking for an ADU?
Not if your ADU is under 1,000 sq ft or within a half-mile of frequent transit. Larger ADUs require one or two off-street spaces depending on size and lot area.
What's the maximum size for a detached ADU (DADU)?
1,200 sq ft, with a height limit of 24 ft (28 ft if built above an existing accessory structure).
How much does an ADU cost to build in Bellevue?
Garage conversions run roughly $70,000-$180,000, basement conversions roughly $100,000-$220,000, and new detached units roughly $150,000-$330,000+, based on local contractor estimates and regional reporting - Bellevue projects tend to land toward the higher end given Eastside construction costs.
Does Washington's statewide ADU law change anything for Bellevue specifically?
Yes. HB 1337 (2023) and middle-housing law HB 1110 required Bellevue to allow detached ADUs and up to two units per lot - something the city hadn't previously permitted. Bellevue adopted Ordinances 6851 and 6852 in 2025 to comply.
Can I rent my ADU out on Airbnb or a short-term rental platform?
No. Bellevue restricts ADU rentals to long-term occupancy of 30 days or more; whole-unit short-term rental of an ADU isn't permitted.
