ADU and DADU Rules on the Eastside: Bellevue, Kirkland, Redmond, Sammamish (2026 Guide)
Quick Answer
Since the 2025 code updates, every Eastside city in this guide allows two accessory dwelling units on a single-family lot: two detached, two attached, or one of each. Washington law (RCW 36.70A.681) stops cities from requiring the owner to live on site, from capping an ADU below 1,000 sq ft, from limiting its height below 24 ft (unless the main house is lower) and from charging more than half the impact fees of a new house.
What still differs by city is the size cap (1,000 sq ft in Redmond, Sammamish, Issaquah and Renton, 1,200 sq ft in Bellevue, Kirkland and Bothell), setbacks, parking and fees. Cost is driven by the type of unit: in Seattle's own survey the median detached ADU cost $230,000 and the median attached ADU $100,000 (projects permitted 2016-2021, so today's prices are higher).
ADU, DADU, AADU: What the Terms Mean
- ADU (accessory dwelling unit) is a second, smaller home on the same lot as a house, with its own kitchen, bathroom and entrance.
- DADU (detached ADU) is a separate building in the yard, often called a backyard cottage. A converted detached garage is also a DADU.
- AADU (attached ADU) is inside or attached to the house: a basement apartment, a unit over the garage, or an addition.
The type you choose decides most of the budget. A DADU needs its own foundation, roof, utility runs and often a long sewer connection. An attached unit reuses the house's structure and services.
What Washington State Law Requires From Every City
House Bill 1337 (2023) rewrote the ADU rules for cities inside an urban growth area. The law now sits in RCW 36.70A.680 and RCW 36.70A.681. For a single-family lot it means:
| Rule | What cities must allow |
|---|---|
| Number of ADUs | At least two per lot: 1 attached + 1 detached, 2 attached, or 2 detached |
| Maximum size | A city may not cap an ADU below 1,000 sq ft |
| Height | A city may not limit an ADU below 24 ft, unless the main house's limit is lower |
| Owner occupancy | A city may not require the owner to live in either unit |
| Parking | None may be required within 1/2 mile walking distance of a major transit stop; elsewhere no more than 1 space per unit on lots under 6,000 sq ft, 2 per unit on larger lots |
| Impact fees | No more than 50% of the impact fees charged for the main house |
| Setbacks and design | No stricter than for the main house; a DADU can sit on a lot line that faces an alley |
| Garage conversions | Existing structures, including detached garages, can be converted even if they break current setback or lot-coverage rules |
| Sale | A city may not prohibit selling an ADU as a condominium unit |
Cities had to adopt these rules with their comprehensive plan updates; Bellevue and Renton adopted theirs in June 2025. Where a local rule still conflicts with the state law, the state law wins.
City-by-City Rules on the Eastside
Codes change, and a few cities are still publishing their final versions online. Use the table to plan, then confirm on the city page linked below before you pay for design.
| City | ADUs per lot | Max size | DADU height | Notes |
|---|---|---|---|---|
| Bellevue | 2 | 1,200 sq ft | 24 ft (28 ft over an existing accessory building) | No owner-occupancy rule since 2023 |
| Kirkland | 2 | 1,200 sq ft | Same as the house in that zone | Pre-approved DADU plans, royalty up to $1,000 |
| Redmond | 2 ADUs or tiny homes | 1,000 sq ft (tiny home 400 sq ft) | Check with the city | Rules moved into the Redmond 2050 code |
| Sammamish | 2 | Detached 1,000 sq ft; attached up to 50% of the house or 1,000 sq ft, whichever is greater | 24 ft | No extra parking if the lot already has 4 spaces |
| Bothell | 2 | 1,200 sq ft | 30 ft (33 ft over an existing accessory building) | Conversions can be larger, up to 40% of the house |
| Issaquah | 2 | 1,000 sq ft | Same as the house | Main house must be at least 25% larger than the largest ADU |
| Renton | 2 | 1,000 sq ft | Check with the city | Free pre-designed ADU plans with expedited review |
Bellevue
Bellevue's 2025 amendments to LUC 20.20.120 allow up to two ADUs on any lot that allows a single-family home, each up to 1,200 sq ft. The owner-occupancy requirement was removed in 2023 (bellevuewa.gov/ADU-reform). Bellevue is now preparing up to 10 pre-approved plans for backyard homes and cottages, sold at a set price to shorten permitting; the program is not open yet.
Kirkland
Kirkland allows two ADUs per lot, each up to 1,200 sq ft of gross floor area. Space with a ceiling under 5 ft and porches or decks up to 200 sq ft do not count toward that limit. Height and setbacks follow the zone of the main house. Kirkland runs a pre-approved DADU program: you buy a ready design from a listed architect for a royalty of up to $1,000 and skip most of the plan review.
Fees changed in July 2025: Kirkland now charges impact fees by floor area, and ADUs over 800 sq ft are charged starting in 2026. The city's own example for a 500 sq ft addition is $2,520 park, $2,195 transportation and $365 fire impact fees (Kirkland notice). Our Kirkland ADU page covers the local details.
Redmond
Redmond allows two ADUs or tiny homes on a residential lot: an ADU up to 1,000 sq ft and a tiny home up to 400 sq ft. The rules now live in the Redmond 2050 zoning code. Ask the city about height and parking for your zone before design, because older versions of these rules are still indexed online.
Sammamish
Sammamish allows up to two ADUs on a lot with a detached house. A detached ADU can be up to 1,000 sq ft and 24 ft tall; an attached ADU can be up to half the house or 1,000 sq ft, whichever is greater. In the R-1 zone a detached ADU needs 20 ft from the front lot line and 5 ft from the rear and sides. No additional parking is required if the lot already has four spaces, garage and driveway included.
Bothell, Issaquah and Renton
- Bothell (BMC 12.14.135): up to 1,200 sq ft; a conversion of existing space can be larger but not more than 40% of the house; a DADU can be 30 ft tall, or 33 ft on top of an existing accessory building.
- Issaquah (IMC 18.500.090): two ADUs in any combination, up to 1,000 sq ft each, and the main house must stay at least 25% larger than the largest ADU.
- Renton: up to 1,000 sq ft, no owner-occupancy rule, both units can be rented (Renton HB 1337 page). Renton's Permit Ready ADU Program gives free base plans for 8 models from 415 to 1,000 sq ft with expedited review.
What an ADU Costs Around Seattle
The most reliable local numbers come from the City of Seattle, which surveys owners of permitted ADUs. In its 2022 ADU report, covering units permitted 2016-2021, total cost including design, labor, materials and permits was:
| Type | Median cost | Range reported |
|---|---|---|
| Detached ADU (DADU) | $230,000 | $20,000 to $650,000 |
| Attached ADU (AADU) | $100,000 | $10,000 to $800,000 |
| All ADUs | $200,000 |
Construction prices have risen since those projects were built, so treat the medians as a floor for a new DADU. For planning on the Eastside, our own starting figures for 2026 are on the ADU construction page:
- Studio DADU, about 400 sq ft: from about $180,000
- One-bedroom DADU, about 600 sq ft: about $240,000 to $330,000
- Two-bedroom cottage, about 800 sq ft: from about $300,000
- Garage conversion, about 400-500 sq ft: from about $95,000, because the foundation and roof already exist
Every figure is preliminary until a site visit. The items that move a budget most are:
- Utilities. Many ADUs share the house's water and sewer and take a sub-panel from the existing electrical service. A separate service line adds roughly $8,000-$18,000 depending on trench length.
- Sewer capacity charge. In King County a new ADU adds a monthly sewer capacity charge: in 2026 an ADU counts as 0.59 of a residential unit, about $46 per month for 15 years, and it can be paid off early at a discount (King County).
- Impact fees. Capped at half of a new house's fees by state law; Kirkland's example above shows the scale.
- Site. Slope, access for equipment, trees and the distance to the sewer main can add more than the finish level.
How Long It Takes
Permitting is the part you can shorten. Seattle measured it for 2022: a DADU on a pre-approved plan took 55.7 days on average from application to permit, while a custom DADU took 149 days (Seattle 2022 ADU report). That is why Kirkland's and Renton's ready-made plans, and Bellevue's upcoming ones, matter: they remove most of the review cycles.
A realistic sequence for a backyard cottage:
- Site visit, feasibility and budget: 1-2 weeks
- Design or choice of a pre-approved plan: 2-8 weeks
- Permit review: about 2 months on a pre-approved plan, longer for a custom design
- Construction: several months for a new DADU, less for a garage or basement conversion
How Homeowners Pay for It
In Seattle's survey, more than 60% of ADU owners paid at least partly in cash, and a home equity loan or HELOC was the next most common source. Construction loans and renovation mortgages are also used; Fannie Mae's HomeStyle Renovation loan can fund adding an ADU to your own home, with one ADU per property under its rules. Talk to your lender before design, because the loan type can decide whether the ADU needs its own utility meters.
An ADU can also be sold separately as a condominium in Washington. In Seattle, ADUs sold this way had a median price of $757,500, against $1.18 million for the main houses (Seattle 2023 ADU report).
Before You Hire a Designer: Checklist
- Confirm your lot is zoned for single-family homes and is inside your city limits, not unincorporated King County.
- Measure what you have: lot size, slope, existing parking spaces, distance from the house to the street sewer.
- Check the size cap and height for your city in the table above, then on the city's page.
- Ask whether your city has pre-approved plans (Kirkland and Renton do; Bellevue is preparing them).
- Decide early between shared and separate utilities.
- Get a written budget that lists permit fees, impact fees and utility work separately.
Frequently Asked Questions
Can I build two ADUs on my lot in Bellevue or Kirkland?
Yes. Both cities allow two ADUs on a lot that allows a single-family house, each up to 1,200 sq ft. They can be two detached units, two attached units, or one of each.
Do I have to live on the property to rent out an ADU?
No. Washington law no longer lets cities require the owner to live in the house or the ADU, and Bellevue dropped its rule in 2023.
Can I convert my detached garage into an ADU?
Usually yes. State law lets you convert an existing structure, including a detached garage, even if it does not meet today's setback or lot-coverage rules. A conversion reuses the foundation and roof, which is why it costs far less than a new cottage.
How long does a DADU permit take?
It depends on the city and the plan. In Seattle in 2022 a pre-approved DADU plan averaged about 56 days to permit and a custom design about 149 days. Pre-approved plans in Kirkland and Renton shorten review the same way.
Do I need extra parking for an ADU?
Not within half a mile of a major transit stop. Elsewhere a city may require no more than one space per unit on a lot under 6,000 sq ft. Some cities waive it further: Sammamish requires nothing extra if the lot already has four spaces.
Plan Your ADU With TopVolk
TopVolk Construction is a licensed and insured general contractor in Bellevue (WA L&I license TOPVOCL767CW). We build detached ADUs, garage conversions and attached units across the Eastside, including on Kirkland's pre-approved DADU plans. Start with a free site assessment: call (206) 591-1096 or see our ADU construction page.