ADUs in Bellevue, WA
A detached ADU in Bellevue costs $320,000 to $520,000 all-in, a daylight-basement unit $150,000 to $300,000 and a garage conversion $125,000 to $225,000. Bellevue's larger lots give more room for backyard cottages than most of Seattle, and its many daylight basements are natural candidates for attached units.
Bellevue ADU costs
| ADU | Typical Bellevue range | Bellevue factors |
|---|---|---|
| Detached ADU, 600 – 1,000 sq ft | $320,000 – $520,000 | Lot size, utilities, trees |
| Daylight basement ADU | $150,000 – $300,000 | Egress, drainage, fire separation |
| Garage conversion | $125,000 – $225,000 | Attached garages need rated separation |
How state ADU law applies in Bellevue
Washington law requires Bellevue, like other cities in urban areas, to allow at least two ADUs on most residential lots, without an owner-occupancy requirement and with limits on parking requirements and impact fees. Bellevue's own code sets the details on size, height and placement. Check the current standards for your lot before design.
What Bellevue lots bring to an ADU
Daylight basements on hillside lots already have a walk-out side, which makes a separate entrance and egress windows straightforward. Drainage on the uphill wall must be sound first.
Larger lots in central and northeast Bellevue can fit a detached unit behind the house without crowding the yard.
Trees. Significant-tree retention rules can determine where a detached unit goes on wooded lots.
Streams and slopes. Critical areas and their buffers in Lake Hills, Kelsey Creek, Coal Creek and the hillside neighborhoods may limit or shape an ADU.
Bellevue ADU permits
An ADU needs a building permit with full drawings, energy compliance and utility plans through MyBuildingPermit, plus trade permits and often tree and stormwater review. Sewer and water connections are reviewed by the utility.
Bellevue ADU questions
Can I convert my daylight basement into an ADU?
Often, if it has enough ceiling height, can meet egress and fire separation, and stays dry.
Does Bellevue require parking for an ADU?
State law limits parking requirements, especially near frequent transit. Bellevue's code sets the rest.
Can I build a two-story backyard cottage?
Where height limits and the lot allow. Many Bellevue lots can fit a compact second floor or loft.
How long does a Bellevue ADU take?
Twelve to eighteen months for a detached unit is typical.