Your permit might issue in October, just as the rain settles in. That does not mean the project sits idle for months. Seattle DADU weather changes how crews protect the site, schedule earthwork and keep materials dry, and city rules get stricter in the wet months. Here, DADU means detached accessory dwelling unit, a separate home on the same lot as a house.

The short version: winter work is allowed, but crews must cover bare soil much faster.

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

Can You Build Through the Wet Season?

Yes. Construction continues year round in Seattle, and each season brings different tasks. Dry months can suit earthwork and foundations. Wet months call for more cover, drainage control and care with open materials.

The weeks ahead of closing in the structure leave the most work exposed to rain. Planning earthwork, the foundation and the roof around likely weather can reduce that exposure. The right approach depends on the site and the design.

Wet Season Soil Rules

Seattle’s clearest weather rule comes from its 2026 Stormwater Code. Under SMC 22.805.020, crews must stabilize all exposed and unworked soils, including stockpiles.

• October 1 to April 30: exposed and unworked soil has a two-day limit.
• May 1 to September 30: the limit is seven days.

Crews must also stabilize soil at the end of the shift ahead of a holiday or weekend if the forecast calls for it. Stockpiles need protection from erosion, sediment trapping and a spot away from storm drain inlets and waterways.

Erosion Control Comes First

Rain moves loose soil. Erosion control goes in first, because Seattle’s residential code calls for a first ground disturbance inspection once crews install those controls.

Also, the city’s construction stormwater manual lists tools for covering bare ground, such as temporary seeding, mulching, matting and compost blankets. It calls clear plastic covering the least preferred cover practice. The post on DADU site drainage explains how these measures fit site work.

Many cottage projects can use the city’s standard plan for construction stormwater control and soil management. SDCI says it fits projects under 5,000 square feet of new and replaced hard surface or land disturbance.

Keeping Streets and Drains Clean

Wet weather carries mud off the site on tires and boots. If crews track sediment off site, the code says they must clean roads thoroughly at the end of each day. During wet weather, that cleaning must happen at least twice a day.

Street washing is allowed only after the sediment is removed, and the wash water cannot enter the drainage system. A planned access route helps, and the post on DADU construction access covers routes and staging.

Concrete in Cold or Wet Conditions

Foundations involve concrete, which reacts to temperature and moisture. Cold or wet conditions can change how a pour is scheduled and protected. The engineer, the concrete supplier and the plans set the methods for each project, so ask how fresh concrete will be protected if the forecast turns.

A concrete washout area practice in the stormwater manual that keeps washout water out of drains. For foundation basics, read about DADU foundation types.

Protecting Framing and Materials

Lumber, sheathing and insulation need to stay dry, so crews usually store them off the ground and under cover. The manual has a practice for material delivery, storage and containment, which aims to keep pollutants out of runoff.

Once framing starts, the goal is to close in the structure. A roof and wall wrap reduce exposure for the work that follows.

Does Rain Change the Inspection Order?

No. Section R106.4 of the residential code says work must stay accessible and exposed until the building official approves it. A crew cannot cover framing or insulation early just because rain is coming.

Temporary protection has to let inspectors see the work. The insulation inspection happens once insulation and vapor barriers are in place but ahead of any gypsum board. Keeping that work dry until the visit is part of the plan, and interior work after it depends far less on the weather.

A Hypothetical Maple Leaf Winter Start

Picture an invented example: a Maple Leaf cottage with excavation set for early December. The crew stages straw mulch and covers on site ahead of digging. Each afternoon, any soil that will sit idle past the limit gets covered.

A gravel pad at the driveway limits tracking, and the crew sweeps the street twice on rainy days. Nothing here describes a real property, but it shows how the wet season rules turn into a daily routine.

Common Wet Season Surprises

• Stockpiles left uncovered over a weekend.
• Mud tracked onto the street after a storm.
• Insulation soaked while it waits for inspection.
• Concrete pours pushed by a cold snap.
• Wash water routed toward a storm drain.

Wet Weather FAQ

Does Seattle ban earthwork in winter?

No. The Stormwater Code allows it but shortens the cover deadline. In the wet months, exposed and unworked soil gets two days. The summer limit is seven days.

Is plastic sheeting allowed for cover?

It is listed, but the manual ranks clear plastic last among cover options. Seeding, mulch, matting and compost blankets are other listed options. Ask which method suits each area of the site.

Who checks erosion controls after heavy rain?

The contractor maintains them on site. An early ground disturbance inspection confirms the method at the start. Ask your builder who will walk the site after storms and how often.

Wet Season Planning Checklist

• Confirm how the crew will meet the two-day cover limit.
• Mark where stockpiles and materials will sit.
• Plan a stabilized access point at the street.
• Set up a concrete washout area.
• Keep insulation dry for the inspector’s visit.

Seattle DADU weather is a planning factor, not a stop sign. Settle the soil cover plan and the access route early, and the rest of the wet season gets easier. The construction and project management section covers related topics.