Treat a Seattle DADU as a House-and-Site Project

Seattle often uses DADU for a detached accessory dwelling unit, or backyard cottage. It can work well for family, rental housing or later needs. It also needs its own foundation, weatherproof shell and planned links to the property’s utilities.

Establish the buildable working area

Start with more than the finished footprint. Show the digging area, tree and building protection, storage and waste area. Add the worker path and room for concrete or lifting equipment. An alley helps only if its width, slope, overhead space and use rules work for the plan.

Ask the builder to name the equipment and delivery method in the price. A narrow side yard may require smaller machines, more labor and a different work order. That changes cost.

Define preconstruction as deliverables

The DADU quote should state who completes each early task:

• property records and the city or county check.
• survey or measured base drawing.
• chosen plan rights and changes.
• architecture, structure and energy work.
• foundation and drainage details.
• utility filings or service planning.
• City filing and correction responses.
• pricing updates when the issued design changes.

“Design and permits included” is not enough. Name the documents, experts, fees and number of City response rounds.

Match the foundation to the site

The plan, soil, slope, drainage and access all affect the foundation. A ready plan may show the building but not how it meets this ground. State who checks the site and who designs any needed change.

The digging price should cover demolition, soil removal or reuse, and temporary protection. Add wet-ground assumptions, steel, inspections and backfill. If retaining walls or soil reports are excluded, state when they would be added.

Specify a Pacific Northwest envelope

The contract should explain how the outside shell will be built. List the roof, drainage, wall layers, windows, doors, flashing, insulation and ventilation. Product allowances need clear quality and installation rules, not just a dollar amount.

Ask who checks water-management details in the field and what evidence is preserved before they are covered. Small buildings have little tolerance for poorly coordinated penetrations and roof-water discharge near foundations.

Coordinate the interior systems

List the heating, cooling, hot water and ventilation systems. Add the electrical panel, plumbing fixtures, lights, internet and appliances. Show how each home can reach its controls and shutoffs.

The finish schedule should name flooring, cabinetry, counters, tile areas, paint, hardware and appliances or confirm realistic allowances. Bidders cannot price the same unit if one assumes rental-grade selections and another assumes custom millwork.

Draw every external connection

Show power, water, sewer and drainage routes on one site plan. Mark the connection points, trench lengths and damage to the driveway or yard. Price utility and public fees separately from builder labor.

Seattle City Light, Seattle Public Utilities, SDCI and King County may each affect parts of the route or charges. The site and utility guide helps allocate these duties.

Use pre-approved plans as one input

An ADUniverse design can reduce repeated design review when its footprint and options work for the site. Before licensing or selecting one, compare orientation, windows, access, base type and allowed changes with the site check record.

Request a list of site sheets and expert work still needed. If important changes would undo the value of the reviewed plan, compare a custom concept before committing. See pre-approved versus custom design.

Construct a complete price

Current Seattle-area company guides do not agree on one number. One publishes $300,000 to $450,000-plus for new DADUs; another publishes $280,000 to $650,000 for new detached work. The cost guide names both sources and their different assumptions. Use the spread for early planning, not as a promised DADU price.

Ask for separate prices for design, experts and public fees. Add site protection, digging, foundation, shell, systems and indoor work. Keep utility links and repairs in their own lines. The cost guide gives every bidder the same list.

Decide whether detachment solves the owner’s problem

A DADU may be a poor fit when the yard is tight or trees and slopes control the plan. Long utility runs can also make it costly. An internal or attached unit may save yard space and reduce ground work.

Conversely, detachment can be valuable when independent access, privacy and reduced alteration of the main home matter most. Compare life after the work, not only the first estimate.

Build the schedule around dependencies

The schedule should show owner choices, surveys, design and permit review. Add utility work, orders, site setup, foundation, dry-in and rough systems. Then show insulation, finishes, inspections and final yard work. Mark slow products and agency steps.

Ask for the assumptions behind each duration and the method for communicating delay. The Seattle ADU timeline separates controllable tasks from outside dependencies.

Verify the builder and the issued set

Check the legal builder in Washington L&I. Ask for recent DADU projects completed by that company or named team. Where allowed, review permit records and ask owners about site care, changes and final approval.

Before work begins, compare the issued permit drawings with the priced revision. Resolve changes in structure, products, utilities and site work in writing. The builder comparison guide provides the full contract review.