ADU Construction Planning in Seattle, WA

Seattle is the site’s core market and needs the deepest local treatment. The City has a mature ADU program, a public plan gallery and substantial official guidance. Those resources make research easier, but they do not turn a property into a standard construction job.

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

What does Seattle’s official ADU page cover?

SDCI’s Accessory Dwelling Unit guidance distinguishes attached and detached units and explains the relevant construction permit routes. It says current residential, building, mechanical, electrical, energy, land-use, environmentally critical-area and shoreline standards 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 property-specific advice from SDCI.

How should a Seattle lot be screened?

Begin with the parcel and existing legal development. Collect the address, parcel number, zoning, lot dimensions and any survey. Then review:

• existing structures and legal units;
• proposed 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; and
• easements or recorded constraints.

ADUniverse’s property-search tool is useful for orientation, not a permit decision. A designer or provider should reconcile mapping with survey and field conditions.

How do ADUniverse pre-approved DADU plans work?

ADUniverse provides City-reviewed DADU designs from local designers. SDCI says the path can be faster and more predictable, and in many cases gives a 2 to 6 week permit indicator for the pre-approved-plan route. That is a plan-review indicator, not a complete project timeline or guarantee.

The City still reviews site-specific zoning, foundation, drainage, utilities and other conditions. The plan owner may charge a licence or design fee, and changes can require additional review. Confirm:

1. current plan and code status;
2. allowed modifications;
3. designer scope and licence terms;
4. foundation and structural assumptions;
5. site-plan and energy documents;
6. utility and drainage design; and
7. correction-response responsibility.

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

Which permits may be required?

SDCI says adding within an existing house, building a detached unit and legalizing an existing unit all require a construction permit through the applicable path. Electrical service changes and other trade work may need separate permits and utility coordination.

A new DADU package commonly needs a site plan, building drawings, structural and energy information, foundation details, drainage information and trade coordination. Conversion work needs accurate existing-condition documentation and may uncover upgrade requirements.

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

What Seattle utility questions belong in feasibility?

Identify the existing electrical panel and service, water line, side sewer, heating system and stormwater path. Ask whether the ADU will use shared or separate equipment and where shutoffs and meters will be located.

Seattle City Light, Seattle Public Utilities and King County wastewater charges are separate parts of the project. A contractor’s building price may not include service upgrades, utility fees, side-sewer work, trenching, restoration or capacity charges.

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

How much should be budgeted in Seattle?

Published 2026 provider guides cover a broad span, often about $150,000 to $500,000 or more, with some detached projects priced higher. These figures combine different unit sizes, site conditions, finishes and definitions of complete.

Use the Seattle ADU cost guide to separate survey and design, permits, site preparation, foundation and building work, utilities, finishes, external restoration and contingency. The address-specific scope overrides a headline range.

What schedule should be requested?

Build a dependency schedule rather than accepting one completion date. Show feasibility, survey, design, owner decisions, permit intake, City review, corrections, utility design, procurement, site work, building inspections and final approvals.

Seattle’s published permit-performance information can provide context, but the project’s own submission 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?

Search the legal contracting entity in Washington L&I’s Verify tool and compare it with the contract. Request recent Seattle ADU projects with addresses or permit identifiers that can be checked, subject to owner privacy. Ask who prepared the design, who answered SDCI corrections and which entity performed construction.

Give each provider the same site information, plan basis and finish schedule. Use the builder comparison guide to normalize registration, project proof, scope, allowances, schedule, payment and written warranty terms.

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

Create one packet before final bids. Include the parcel and zoning record, current survey or measured site plan, existing-unit information, selected concept, tree and access notes, side-sewer and utility information, finish schedule and any SDCI or utility correspondence. If an ADUniverse design is being used, identify the exact plan, version and approved options.

Every provider should state which packet revision it priced. Ask for a responsibility table covering site plan, foundation adaptation, energy documents, drainage, utility applications, permit corrections, inspections and closeout. This makes a design-build total comparable with a builder-only proposal and exposes work that would otherwise appear later as an allowance.

Before construction, reconcile the issued permit set with the contract. Record any City-driven change to structure, equipment, site work or finishes and agree on price and schedule effects in writing.