You have a permit in hand, or close to it, and the next question is how long the build will take. The honest answer is that no single number fits every lot. A DADU build timeline depends on the site, the design and several steps that other agencies control. Here, DADU means detached accessory dwelling unit, a separate home on the same lot as a house.

What you can know ahead of time is which factors shape the schedule, and each one below comes from a city code or an official agency page.

Details below reflect official Seattle and utility pages checked in October 2026. Confirm with city staff for your property.

Why No Two Schedules Match

Lot slope, soil, access, trees, utilities and design all differ from one project to the next. A one story slab cottage on a flat lot moves through different steps than a two story unit on a hillside.

Much of the schedule depends on others. Inspectors, utilities and suppliers each run their own processes. For the review period ahead of construction, read about the DADU permit timeline.

What Limits Does the Permit Set?

Under Section R105.13 of the Seattle Residential Code, authority to do the work expires 18 months from the date of issuance. An approved renewal extends the permit for another 18 months from the prior expiration date.

This rule does not say how long a build takes. It sets an outer limit the project must respect, so steady work and a marked renewal date protect the permit.

How Inspections Shape the Pace

Construction moves in steps, and each step needs approval. The code lists inspections for ground disturbance, foundation, slab or under floor work, framing, insulation and final. Section R106.6 says no work may go past each point without the building official’s approval.

The SDCI construction inspections page explains timing. If you call the request line before 7 a.m., SDCI usually inspects the same day. Later calls go on the next workday’s list.

A failed inspection adds time, since the crew must correct the work and request another visit. The post on DADU building inspections walks through each one.

Utility Steps Run on Their Own Clock

Power, water, sewer and gas each follow a separate path. City Light’s new or upgraded service page says to allow 4 to 6 weeks for initial processing after you apply. An incomplete application may take longer.

That figure covers only the first review, not the full service work. It does show why utility applications belong early in the plan. Plumbing and gas piping get their own inspections through Public Health Seattle and King County.

Does Weather Change the Schedule?

It can. Under SMC 22.805.020, from October 1 to April 30, no soils may stay exposed and unworked for more than two days. From May 1 to September 30, the limit is seven days.

These limits do not stop winter work. They do require crews to cover or stabilize soil quickly in the wet months, which takes more planning and materials. Crews must also stabilize soil at the end of the shift ahead of a holiday or weekend if the forecast warrants it.

Materials and Lead Times

Windows, doors, cabinets and some mechanical equipment can need advance orders. If a key item arrives late, the next stage may have to wait.

A clear finish list helps you order long lead items early. The post on DADU finish allowances explains how finish choices tie into the budget.

Changes After the Permit Issues

Design changes can affect the schedule. Some changes to an issued permit need a plan revision that the city reviews first. That review takes time and may add fees.

Settling the design before construction starts is one of the few schedule factors fully in your control. Small field changes still come up, and a DADU contingency budget helps absorb them.

Working Hours and Site Limits

Seattle’s noise rules shape the workday. Outside multifamily and commercial zones, SDCI says construction may run from 7 a.m. to 10 p.m. on weekdays. On weekends and legal holidays, it starts at 9 a.m. and ends at the same hour.

Impact work such as jackhammers has a shorter window, from 8 a.m. until 5 p.m. on weekdays, with a 9 a.m. start on weekends and holidays. Narrow access, street use permits and tree protection can also shape how crews work each day.

A Hypothetical Delridge Hillside Build

Picture an invented example: a Delridge cottage on a sloped lot with a narrow side yard. Excavation starts in November, so the crew plans covers for every exposed patch of soil. Utility applications go in while the design is still in review.

The owner picks windows and cabinets ahead of framing, giving the orders time to arrive. Nothing here describes a real property, but it shows how early choices take pressure off the schedule.

Common Schedule Surprises

• Utility applications sent in late or incomplete.
• Failed inspections that need a return visit.
• Wet season earthwork that needs extra covers.
• Late selections for windows and cabinets.
• Design changes that need a plan revision.

Build Timeline FAQ

Can anyone promise how long my cottage will take?

Not reliably. Inspections, utilities, weather and supply timing all sit partly outside any one person’s control. A builder can share a planned sequence, but treat any total as an estimate and ask what assumptions sit behind it.

What happens if the permit expires mid build?

Section R105.13 allows reestablishment of an expired permit in some cases. The rules for renewal and reestablishment differ, so ask SDCI ahead of the expiration date.

Can crews work on weekends?

Yes, within limits. In most residential areas, weekend and holiday work can begin at nine in the morning. Louder impact work must stop by five, and the zone rules are on SDCI’s noise pages.

Schedule Planning Checklist

• Note the permit issue date and the 18 month limit.
• Submit utility applications early and completely.
• Plan erosion covers if earthwork falls in the wet season.
• Order long lead items once selections are final.
• Lock the design to limit plan revisions.

Get utility applications in early and settle the design before the first inspection. Those two moves do the most to steady a DADU build timeline. The construction and project management section covers related topics.