Looking for an ADU builder in Mercer Island, WA? Describe your project once and up to three licensed ADU builders who work in Mercer Island and King County get back to you with a quote. It's free and there is no obligation.
Mercer Island is home to about 25,302 people (2024 Census Bureau estimate), a shrinking community (−1.8% since 2020). With 3,964 people per sq mi, it is a densely built-up area. Homeowners here add accessory dwelling units for rental income, family members or a home office.
Mercer Island at a glance
- Population (Census Bureau estimate, 2024)
- 25,302 (−1.8% since 2020)
- Rank in Washington
- 45th of 281 cities and towns
- Land area
- 6.4 sq mi
- Population density
- 3,964 people per sq mi (densely built-up)
- Type
- City (Mercer Island city)
- Main county
- King County
What to check before hiring in Mercer Island
- Rules: ADU rules are set by the state and the city; check Mercer Island's planning department for size, setback and parking limits.
- License and permit: in Washington, ADUs are built by licensed contractors under a building permit.
- Utilities: water, sewer and electrical capacity can drive the cost as much as the unit itself.
- Compare: ask for references and photos of ADUs each builder has completed.
Useful official resources
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HUD User
Federal housing research, including ADUs.
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ICC: International Code Council
Publisher of the building codes most states and cities adopt.
Services available in Mercer Island
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Detached ADU
Design and construction of a detached backyard unit, site-built or prefab, with the permit and utilities.
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Garage or interior conversion
Conversion of a garage, basement or part of the house into a legal accessory dwelling unit.
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ADU plans and permit
Feasibility, plans and permit application for an ADU before choosing a builder.
Frequently asked questions in Mercer Island
How much does an ADU cost in Mercer Island, WA?
It depends on the type of unit, its size, the finishes and the utilities. Describe your project: builders who serve Mercer Island reply with a quote.
How long does an ADU take?
Often several months for permits and a few more for construction; conversions and prefabs can be faster.
Do I need an architect?
Not always: designers, builders with in-house design or prefab companies can prepare the permit set, depending on local rules.
Which licensed ADU builders will contact me?
Up to three independent licensed ADU builders who serve Mercer Island and the surrounding area, chosen for the service you need. Your details are shared only with your consent.
Source: U.S. Census Bureau, Population Estimates Program (Vintage 2024) and Gazetteer Files (public domain), imported on 10/31/2026. Figures describe the incorporated place or county as published; they are shown for information only.