What Does an ADU Cost in Seattle in 2026?

Updated 6 August 2026. Published Seattle provider guides use different assumptions. Seattle ADU Inc publishes a broad $150,000–$500,000 range, while Seattle ADU Builders publishes substantially higher starting ranges for several detached and attached paths. These are commercial estimates, not audited government price data, and neither source defines the same property or complete scope.

The disagreement is useful: there is no honest one-price answer without a parcel and a written scope. This independent site does not provide construction quotes.

First identify the project type

A garage conversion may reuse walls and a roof, but it can require slab, framing, moisture, fire, energy and utility upgrades. A detached ADU needs a complete foundation, structure and envelope. An attached or second-story unit adds connection and protection work around an occupied home. Steep-slope, shoreline, critical-area, tree and limited-access sites need separate treatment.

Do not compare a small interior conversion with a ground-up two-bedroom unit by square-foot rate alone.

Build the budget in nine sections

1. Feasibility and property information

Allow for records research, jurisdiction confirmation, measured site information, survey work where needed, title or easement review, early utility checks and specialist advice. Spending here can prevent a plan from being designed around the wrong assumptions.

2. Design, engineering and energy documents

Identify architectural design, structural engineering, energy compliance, civil or drainage input, soils or geology work where needed, mechanical/electrical/plumbing documents and revisions. Ask whether visualisation, interior selections and permit corrections are included.

3. Applications, plan check and agency charges

Separate government and utility charges from the provider’s permit-administration fee. Record who pays each invoice. City and county processes differ, so do not import one jurisdiction’s fee estimate into another.

4. Demolition, access and site preparation

Include removals, protection of the occupied property, temporary facilities, staging, hauling, excavation, grading, shoring or retaining work, soil handling and restoration. Narrow access can turn ordinary work into labour-intensive work.

5. Foundation, structure and exterior envelope

This section covers concrete or another approved foundation system, framing, roof, windows, exterior doors, weatherproofing, insulation, cladding and exterior coatings. Verify whether gutters, drainage connections, decks, steps and exterior lighting are included.

6. Building systems and utilities

List electrical service and panel work, wiring, HVAC, ventilation, plumbing, water heating, sewer, water, gas if proposed, fire sprinklers if required and communications. The utilities guide explains why trench length and existing capacity matter.

7. Interiors, fixtures and appliances

Use a written finish schedule. Cabinet length, counter material, tile areas, plumbing brands, lighting quantities, flooring, doors, trim, paint, appliances and hardware can create large allowance differences. “Standard finish” is not a specification.

8. External work and property restoration

Include paths, fencing, gates, planting, irrigation repair, drive or paving repair, drainage, refuse areas and work around the main house. A proposal that stops at the ADU wall may not deliver a usable finished property.

9. Contingency and owner costs

Keep a contingency suited to the stage and uncertainty. Existing structures and buried services carry more unknowns than a fully investigated open site. Also record financing, insurance, temporary accommodation, owner consultants and owner-supplied items separately.

Why price per square foot misleads

A kitchen, bathroom, electrical service and utility connection create substantial fixed costs. A smaller unit can therefore cost more per square foot. Published rates may include only construction, while others include design and permits. Site work may be excluded or carried as an allowance.

If a provider gives a rate, ask for its numerator and denominator: total contract amount, floor area used, and every included or excluded category.

Normalize three proposals

Washington L&I’s hiring guidance tells owners to verify contractor registration, bond, insurance and history and to use a written contract. For a six-figure ADU, collect several written proposals using the same drawings, specifications and work. Transfer them into a side-by-side schedule with one row for every budget section above.

For each proposal record:

• exact plans and revision date priced;
• fixed amounts versus allowances;
• permit, utility and agency charges;
• exclusions and owner-supplied work;
• payment milestones;
• change-order markup and concealed-condition process;
• start and completion assumptions; and
• warranty and closeout deliverables.

The lowest total may simply have the most unpriced rows.

Treat deposits and changes carefully

Washington L&I advises owners to use a written contract and understand payment terms. The agreement should define scope, materials or allowances, permit responsibility, payment milestones and how changes are documented. Do not import California down-payment rules into a Washington contract; check current Washington requirements and obtain legal advice where appropriate.

Tie later payments to clear work or delivery milestones. Do not let a payment schedule get far ahead of completed work.

A better early budget statement

Instead of “my ADU costs $250,000,” use: “I am testing a detached one-bedroom unit of roughly this size on this address, with these access and utility assumptions, these finishes and a separate contingency.” That statement can be corrected as evidence improves.