What does accessory dwelling unit mean

Looking for what does accessory dwelling unit mean in Seattle, WA? Call AccessoryDwellingUnit Pros for a contact a provider — no obligation. Licensed adu builders serve Seattle and the surrounding area, and you'll get straight answers on timing and what affects the price.

Last updated: September 24, 2026

What affects the price

Building an ADU is a custom project, so costs vary based on your specific choices. Site conditions are the biggest factor; if your land requires significant grading, utility extensions, or foundation work, those expenses add up quickly. Choosing high-end finishes, custom cabinetry, or complex roof lines will also push the budget higher compared to a standard design. Prefabricated units often offer predictable costs, while custom site-built structures allow for more flexibility but can be harder to estimate upfront. Exact pricing confirmed free on the call.

About this service

An accessory dwelling unit is a secondary, self-contained living area located on the same lot as a primary residence. It includes its own kitchen, bathroom, and sleeping area. You might hear these referred to as granny flats, carriage houses, or backyard cottages. You need to know this term when researching your local zoning codes, as it is the official legal definition used by building departments to regulate your project. Exact pricing confirmed free on the call.

Frequently asked in Seattle

What is the difference between an ADU and a basement apartment in Seattle?

An accessory dwelling unit (ADU) in Seattle is a secondary housing unit on a single-family lot, often with its own entrance and utilities. A basement apartment might be part of the primary dwelling, sharing main systems, unless specifically converted and permitted as a legal ADU. Both must meet Seattle's building and zoning codes for habitability and safety.

How can I get an estimate for a local ADU project in Seattle?

What are the best accessory dwelling unit options for Seattle homeowners?

The best accessory dwelling unit (ADU) options for Seattle homeowners depend on their property layout, budget, and intended use. Options include attached ADUs (like basement or attic conversions), detached ADUs (new backyard structures), and garage conversions. Each must comply with Seattle's building codes and zoning regulations for size, setbacks, and access.

What are the costs involved in building an accessory dwelling unit in Seattle?

The cost to build an accessory dwelling unit (ADU) in Seattle is influenced by factors such as size, design complexity, foundation type, and the chosen materials. Site preparation, utility connections, and finishing details also contribute to the overall expense. Obtaining detailed quotes from experienced ADU builders is essential for accurate budgeting.

Where can I find reputable accessory dwelling unit contractors in Seattle?

What is involved in getting an accessory dwelling unit permit in Seattle?

Obtaining an accessory dwelling unit (ADU) permit in Seattle involves submitting detailed architectural and structural plans to the City of Seattle's Department of Construction and Inspections. These plans must demonstrate compliance with all applicable zoning ordinances and building codes. The review process ensures the proposed ADU meets safety and habitability standards before construction begins.

Are there services for affordable accessory dwelling units in Seattle?

While all ADU projects involve significant investment, exploring options for affordable accessory dwelling units in Seattle can involve careful planning and design choices. Some programs or incentives may be available through city initiatives. Focusing on efficient layouts and standard materials can also help manage overall construction costs for your ADU project.

Also serving nearby: Kirkland · Bellevue · Renton · Kent · Federal Way

All cities in Washington →

More on this: HUD ADU resources — ADU policy and financing research.

Service area details

755,078residents

8,989per sq mile

8ZIP codes served

Serving all of Seattle — population 755,078; about 8,989 residents per square mile across 84.0 sq mi. ZIP codes covered include 98101, 98102, 98103, 98104, 98105, 98106, 98107, 98108.

Neighborhoods served in Seattle: Ballard, Fremont, Capitol Hill, Queen Anne, Green Lake, West Seattle — and every surrounding area.