ADU Rules in Bellevue (Including Factoria)

Bellevue allows two accessory dwelling units on any lot where a single-family home is permitted, and since mid-2025 either or both can be detached. Each ADU may reach 1,200 square feet, a backyard unit can be 24 feet tall, and units under 1,000 square feet or near major transit need no parking.

That is a large shift for a city that, until recently, only allowed one unit built inside or onto the main house. This guide walks through the current Land Use Code section on ADUs, the fees and permits that come with a project, and what the rules mean for the split-levels and sloped lots typical of Factoria. For how Bellevue’s approach compares with nearby cities, see our regional overview of HB 1337 ADU rules by city.

Bellevue’s Two-Step Path to Backyard Cottages

The first step came in July 2023, when the City Council adopted Ordinance 6746. It dropped owner occupancy, allowed ADUs to be conveyed as condominium units, raised the size limit, tied parking to the size of the unit and removed separate design controls. It did not address detached units, which remained off the table at that point.

The second step was Ordinance 6851, adopted on June 24, 2025 as part of Bellevue’s middle housing package. It rewrote Land Use Code section 20.20.120 so that it now regulates both attached and detached ADUs. A companion ordinance, 6852, amended other parts of the city code, including impact fees and street frontage rules.

One practical warning: summaries written before mid-2025 still describe Bellevue as an attached-only city. If a designer, listing or older article told you a backyard cottage was not possible, that advice is out of date.

Attached, Detached or One of Each

The code permits up to two ADUs on each lot located in a land use district that allows a single-family dwelling on that lot. The two can be any mix: a basement unit and a cottage, two attached units, or two detached buildings.

An attached ADU must sit inside the main house or be attached to it, and it follows the same height and setback rules as the house. A detached ADU is a separate building with its own standards, covered below.

Why a Backyard Unit Uses Up a Middle Housing Slot

Under Bellevue’s middle housing standards, a lot has a maximum number of dwelling units, generally four, with more allowed in some situations such as proximity to major transit. Detached ADUs count toward that number; attached ADUs do not. If you are weighing a cottage now and a duplex or cottage housing later, plan the count from the start.

Lots Created by Unit Lot Subdivision

If your lot was created through a unit lot subdivision or unit lot short subdivision and was built out under that approval, the code does not allow a new ADU on it. This matters for some newer townhome-style developments.

Bellevue’s Floor Area Limit and What It Exempts

Each ADU is limited to 1,200 square feet of floor area. Up to 300 square feet per ADU that is used for parking or unheated storage is exempt from that limit, which makes a cottage with a single-car garage bay or a storage room workable without shrinking the living space.

The Director can approve a larger unit in a few situations: an ADU that occupies a single floor of the main house, an addition to an existing detached structure, or the conversion of an existing detached structure. A building previously permitted as a guest cottage can be converted to an ADU regardless of the size limit, provided the conversion does not expand it.

Floor area ratio rules for the lot still apply, so on a smaller parcel with a large house, the lot-wide limit may cap the ADU before the 1,200-square-foot figure does.

Height and Placement for a Bellevue Backyard Unit

A detached ADU may be up to 24 feet tall, or 28 feet when it is built as an addition over an existing accessory structure, such as a unit above a detached garage. That fits two stories under a modest roof pitch, though it is lower than what Seattle now allows, as our guide to Seattle’s ADU and DADU rules explains.

For setbacks, lot coverage and impervious surface, detached ADUs follow the middle housing dimensional standards in Land Use Code 20.20.538. In the residential districts covered by those standards, side setbacks are 5 feet and rear setbacks are 10 or 15 feet depending on the district, while coverage and impervious limits also vary by district. Where a lot line abuts an alley, a detached ADU may be placed at that line.

When Bellevue Requires a Parking Space

Parking depends on size and location. No off-street space is required for an ADU under 1,000 square feet, or for any ADU whose lot is a half mile or less from a major transit stop. Outside those cases, each ADU needs one off-street space.

This creates a useful design threshold. Keeping a unit just under 1,000 square feet avoids the parking question entirely, while a larger two- or three-bedroom unit outside the transit area needs a space planned into the site. Under state law the half mile is measured as walking distance, not a straight line on a map, so confirm it for your address.

School Fees, Frontage Work and Utility Hookups

Ordinance 6852 set school impact fees for an ADU at 50 percent of what the principal unit would pay, in the school districts for which Bellevue collects these fees. It also excluded ADUs from the city’s street frontage improvement requirements and clarified that adding an ADU is not a commercial use. State law caps other impact fees for ADUs at half of the principal unit’s fee, so ask the city which fees apply to your permit.

Utilities deserve early attention. A detached unit usually needs its own sewer and water connections or branches from the existing lines, and the county wastewater system bills a capacity charge for each newly connected unit. Most of Bellevue is served by city utilities, but confirm the provider for your parcel. Our post on the water, sewer and electrical costs homeowners forget explains what to budget for.

Bellevue Permits and a Rule About Home Businesses

Bellevue’s Development Services department reviews ADU projects, and applications go through the MyBuildingPermit online portal. Bellevue has also used a land use approval to register an ADU within a single-family residence, so ask at intake which applications your project needs.

One Bellevue-specific rule surprises people: a site may not contain both an ADU and a business that requires a Home Occupation Permit. If you run a client-facing business from home, factor that into the decision before you design.

The city has also been developing a pre-approved plans program that includes detached ADUs, so check its status when you start.

Factoria Lots: Split-Levels, Slopes and Daylight Basements

Factoria, in south Bellevue near the I-90 and I-405 interchange, mixes post-war houses, 1980s split-levels, townhomes and newer construction. That variety shapes which ADU route makes sense.

Split-levels and homes with daylight basements often have a lower floor with its own exterior wall and grade-level access. Converting that floor into an attached ADU can be the most efficient option, and because an ADU on a single floor of the main house may be approved above 1,200 square feet, a large lower level does not have to be cut down.

On sloped lots, height is measured from grade, so a downhill cottage may need a different roof form to stay within 24 feet, and retaining walls, stairs and equipment access add steps to the plan. Townhomes built through unit lot subdivisions, as noted above, usually cannot add an ADU at all. Our Factoria remodeling page describes the homes we see most often in the neighborhood.

Checking a Bellevue Address Before You Design

The code tells you the limits; your parcel decides which ones bind. Lot width, slope, the existing footprint, alley access, sewer location and the distance to transit all change the answer. United Signature handles design, permitting and construction for ADUs across the city, and our Bellevue design-build team can review your lot and the options it supports.

FAQ

Were detached ADUs allowed in Bellevue before 2025?

No. Before Ordinance 6851, Bellevue allowed one ADU and required it to be part of the main house’s structure. Detached units became possible when the 2025 amendments took effect.

Can I run a home-based business on a Bellevue lot that has an ADU?

Not one that requires a Home Occupation Permit. The Land Use Code says a site may not contain both an ADU and a business subject to those permit rules, so check how your business is classified before you build.

Does a detached ADU in Bellevue reduce how many middle housing units I can build?

Yes. Detached ADUs count toward the maximum number of dwelling units allowed on the lot under the middle housing standards, while attached ADUs do not.

Can I build an ADU above my existing detached garage in Bellevue?

Usually, yes. A detached ADU built as an addition over an existing accessory structure can be up to 28 feet tall, and the Director may approve extra floor area for additions to existing detached structures. The garage itself still has to support the new load, which an engineer confirms.

Can an old guest cottage on my Bellevue property become a legal ADU?

Often. A structure previously permitted as a guest cottage can be converted to an ADU without meeting the size limit, as long as the conversion does not expand it. It still needs to meet the building, fire and energy codes for a dwelling.