ADU Construction Planning in Seattle, WA

Seattle has a detailed ADU program, a public plan gallery and useful City guidance. Those tools help owners get started. They do not turn a property into a standard building job.

This independent page is not SDCI, ADUniverse, Seattle City Light, Seattle Public Utilities or a contractor. Confirm current rules and choices directly.

What does Seattle’s official ADU page cover?

SDCI’s ADU guidance separates attached and detached units and explains the permit paths. Home, building, electrical, energy, land-use, critical-area and shoreline rules may apply.

The page also directs owners to Seattle’s ADUniverse and notes the King County sewage treatment capacity charge. Use it as the primary source. Then record any site-specific advice from SDCI.

How should a Seattle lot be screened?

Begin with the parcel and buildings already on the lot. Collect the address, parcel number, zoning, lot size and any survey. Then review:

• existing structures and legal units.
• planned attached, detached or conversion path.
• yards, lot coverage and building placement.
• trees and possible root-protection areas.
• steep slope, shoreline or other mapped conditions.
• alley, driveway and side-yard access.
• sewer, water and electrical service.
• drainage and roof-water path.
• easements or recorded constraints.

ADUniverse’s property tool is useful for an early look, not a permit decision. A designer or builder should compare the map with the survey and site.

How do ADUniverse pre-approved DADU plans work?

ADUniverse provides City-reviewed DADU plans from local designers. SDCI says this path can be faster and often shows a 2-to-6-week permit indicator. That number covers plan review, not the full project, and is not a guarantee.

The City still reviews zoning, foundation, drainage, utilities and other site conditions. The plan owner may charge a fee, and changes may need more review. Confirm:

1. Current plan and code status.
2. Allowed changes.
3. Designer work and license terms.
4. Foundation and structural assumptions.
5. Site-plan and energy documents.
6. Utility and drainage design.
7. correction-response duty.

Use the pre-approved plan comparison before assuming a catalog design is the lowest complete cost.

Which permits may be required?

SDCI says internal units, detached units and work to legalize a current unit need a building permit. Electrical service changes and other trade work may need separate permits and utility planning.

A new DADU often needs a site plan, building plans and structural and energy details. Add the foundation, drainage and trade plans. A conversion needs accurate records of current conditions and may uncover more work.

Read the design and permits guide for a document duty table.

What Seattle utility questions belong in site check?

Find the power panel, water line, side sewer, heating system and rainwater path. Ask what the ADU will share and where meters, equipment and shutoffs will go.

Seattle City Light, Seattle Public Utilities and King County sewer charges are separate parts of the project. A builder’s price may leave out utility upgrades and fees. Side-sewer work, trenches, repairs and capacity charges may also be extra.

The utilities and site-work guide puts those items into the quote schedule.

How much should be budgeted in Seattle?

Two named company guides checked in August 2026 span about $250,000 to $650,000-plus across different Seattle-area ADU types. Their detached ranges also differ. These figures mix sizes, sites, finishes and different ideas of what “complete” means.

Use the Seattle ADU cost guide to separate the survey, design, permits and site setup. Add the foundation, construction, utilities, finishes and outside repairs. Keep money for surprises. The property facts matter more than a headline range.

What schedule should be requested?

Build a schedule that shows which steps depend on others. Include site checks, the survey, design, owner choices and permit intake. Then show City review, corrections, utilities, orders, site work, inspections and final approval.

Seattle’s published permit-performance details can provide context. But the project’s own filing quality, correction cycles, utilities and site work control the actual path. Use the timeline guide to compare assumptions.

How should Seattle ADU builders be checked?

Check the legal builder in Washington L&I’s Verify tool and compare it with the contract. Ask for recent Seattle ADUs with records that can be checked, while protecting owner privacy. Ask who designed the unit, answered SDCI comments and completed the work.

Give each company the same site facts, plans and finish list. Use the builder comparison guide to compare registration, project proof, scope, allowances, timing, payment and warranties.

What belongs in a Seattle permit-and-price packet?

Create one packet before final bids. Include the parcel, zoning, survey or measured site plan and current-unit details. Add the chosen concept, tree and access notes, sewer and utility facts, finishes and agency messages. For ADUniverse, name the plan, version and approved options.

Each company should state which packet date it priced. Ask who handles the site plan, foundation changes, energy records, drainage and utility filings. Add City corrections, inspections and final approval. This makes design-build easier to compare with a builder-only quote.

Before building, compare the issued plans with the contract. Record any City change to the structure, equipment, site or finishes. Agree on the price and timing in writing.