ADU Construction in Seattle Starts With the Lot

An accessory dwelling unit can create a separate home for family, a long-term tenant or future flexibility on property that already has housing. The project may be a detached backyard cottage, an attached addition, a basement conversion or a garage conversion. Each route changes the design, permit, utility and construction work.

This independent guide helps Seattle-area owners prepare a useful project brief and compare providers on the same scope. It is not the Seattle Department of Construction and Inspections, King County or a contractor. The public pages cannot approve a lot, set a final price or promise that a provider is available.

What should be checked first?

1. Which authority reviews the address? Seattle SDCI rules do not automatically apply in Bellevue, Kirkland, Redmond, Shoreline or unincorporated King County.
2. What can the lot support? Existing buildings, setbacks, access, trees, steep slopes, drainage, easements and utility routes can control the design.
3. Which ADU type fits the household goal? A detached build, attached addition and conversion are different projects, not interchangeable price tiers.
4. What is included in the proposal? Feasibility, survey, design, engineering, permits, site work, utilities, finishes and restoration need separate rows.

The ADU feasibility guide turns those questions into an early property checklist.

Choose the Seattle ADU path

Detached ADU or DADU

A detached ADU, often called a DADU or backyard cottage, is a new standalone dwelling. It provides privacy and design flexibility, but it also needs a complete foundation, structure, envelope and building-services scope. Alley or side-yard access, tree protection, sewer routing and construction staging may matter as much as floor area.

Garage or basement conversion

A garage or basement conversion reuses existing space only when the structure and living conditions support the intended work. Slab condition, moisture, ceiling height, insulation, emergency escape, fire separation and utilities can turn an assumed simple conversion into a large alteration or replacement decision.

Attached or internal ADU

An attached ADU may be formed within the house or built as an addition. It can reduce some detached-site work, but it affects the main home. Privacy, sound separation, daylight, stairs, structural changes and shared systems need to be evaluated before comparing its cost with a DADU.

Do Seattle pre-approved plans remove permit work?

No. Seattle’s ADUniverse offers a gallery of pre-approved DADU designs and a property-search tool. SDCI explains that these plans can provide a faster and more predictable review path, but the property still needs site-specific review for zoning, foundation, drainage, utilities and other conditions.

The City’s current ADU guidance says an ADU must meet applicable residential, building, mechanical, electrical, energy, land-use, environmentally critical-area and shoreline requirements. A catalogue image does not answer those questions.

Use the pre-approved plans versus custom design guide to compare plan fit, allowed changes, site work and total installed scope.

What does an ADU cost in Seattle, WA?

Published Seattle-area builder guides commonly place complete ADU projects in a very broad range from roughly $150,000 to $500,000 or more. Some new detached projects are quoted above that range. These are planning figures, not an offer or a verified market average. Garage conversions, basement units and new detached cottages do not share one cost basis.

The Seattle ADU construction cost guide separates:

• feasibility, survey, design and engineering;
• plan review, permits and agency charges;
• demolition, excavation, foundation and access;
• structure, envelope and interior work;
• electrical, water, sewer, heating and ventilation;
• fixtures, appliances and finish allowances;
• drainage, landscaping and property restoration; and
• contingency and owner-supplied items.

A useful comparison starts with one written scope. A low headline price is not comparable if it omits utility work, site restoration, design responses or realistic finish allowances.

Why can utilities and drainage change the plan?

An ADU adds a kitchen, bathroom, electrical load and year-round occupancy. The team should confirm the likely sewer route and condition, water-service approach, electrical capacity, heating system, stormwater treatment and construction access early.

Seattle SDCI also directs ADU applicants to acknowledge the King County sewage treatment capacity charge. Seattle City Light and Seattle Public Utilities have separate service-review processes. Their work is not automatically included in a general building permit or a contractor’s base construction line.

The utilities and site-work guide shows what to record before accepting an all-in price or promised schedule.

How should Seattle ADU builders be compared?

Washington’s Department of Labor & Industries tells homeowners to verify an active contractor registration, bond, insurance and safety or registration history, then use a written contract. Registration is necessary evidence, but it is not proof that a company has completed the same type of ADU on a similar Seattle lot.

Use the ADU builder comparison guide to normalize:

• the legal contracting entity and registration number;
• recent relevant projects and references;
• design, engineering and permit responsibility;
• site, utility and restoration inclusions;
• allowances and owner selections;
• payment milestones and change-order rules;
• schedule assumptions; and
• written warranty terms supplied by the provider.

Which Seattle-area location guide applies?

King County contains Seattle, other incorporated cities and unincorporated areas. They do not share one permit counter or one set of current forms. Start at the Seattle-area service hub, then choose the page for the actual permit authority.

The first release includes a King County jurisdictions guide, an Eastside guide, and researched pages for Seattle, Bellevue, Kirkland, Redmond, Shoreline, Renton, Burien, Bothell and Mercer Island. Those pages explain distinct official resources and early decisions; they do not claim that a provider covers every address.

What makes a useful first enquiry?

Prepare the property address or city and ZIP, the existing building type, the ADU path under consideration, rough room or floor-area goal, access notes, known tree or slope conditions, utility information and any survey or plan documents already available. Say whether the project is at feasibility, design, permit or construction-pricing stage.

Describe the property and project through the Seattle intake form. It supports planning and possible provider routing; it cannot confirm legal buildability or produce a final quote.