Siding and Roofing an ADU in Seattle: What a Backyard Unit's Exterior Needs

A backyard unit is a small house with a full-size exterior problem. It has four walls, a roof, a dozen penetrations and the same eight months of rain as the main house, on a lot where the crew often cannot get a truck within sixty feet of the work. Here is what the outside of a Seattle ADU needs, what it costs, and how a recent job in Northgate went.

What the exterior of an ADU has to do

Three jobs, in order: keep water out, meet the fire and setback rules for where it sits on the lot, and look like it belongs next to the main house. The first two are inspections. The third is what the neighbours and the appraiser notice.

Water. Small roofs have proportionally more edge, valley and penetration per square foot than big ones, and every one of those is a flashing detail. A detached unit also tends to have a kitchen vent, a bath fan, a dryer vent and often a mini-split line set all punching through the same two walls. Housewrap lapped correctly, kickout flashing where the roof meets a wall, and a rain-screen gap behind the siding are what separate a unit that stays dry from one that grows mould behind the drywall in year three. Our guide to fiber cement versus engineered wood siding covers which cladding handles that best here.

Fire and setbacks. Seattle lets a DADU sit close to the rear and side property lines, and the closer it sits, the more the wall assembly has to resist fire. That can mean a rated wall behind the siding, limits on windows in that wall, and a boxed eave instead of an open one. None of it is exotic, but it has to be decided before the siding order, because it changes what the wall is made of. Read how DADUs work in Seattle for the zoning side.

Fit. Match the roof pitch family and the trim proportions of the main house, pick a siding colour that sits beside it rather than against it, and the unit reads as part of the property. Plenty of owners use the ADU build to re-side the main house in the same pass, which is cheaper than two separate mobilisations and makes the lot look finished.

What it costs

For a typical detached unit of 600 to 1,000 square feet in 2026, the exterior package usually looks like this. These are orientation ranges for Seattle and the Eastside, not quotes.

• Siding, trim, housewrap and flashing: $18,000 to $40,000 in fiber cement or engineered wood. Rated wall assemblies on the lot-line side add to that.
• Roof: $8,000 to $16,000 for architectural asphalt with underlayment, flashing and ridge venting; roughly double for standing-seam metal, which suits the low-slope and shed roofs many ADUs use. The roof replacement cost guide explains what moves those numbers.
• Gutters and downspouts: $1,500 to $4,000, plus whatever it takes to get the water somewhere that is not the neighbour’s fence line. Drainage is one of the things a Seattle inspector looks at on a backyard unit.
• Exterior paint or stain: $4,000 to $9,000 if the siding is not pre-finished.

Access is the quiet cost. Material that cannot be delivered next to the unit gets carried through a side yard by hand, and a roof that a lift cannot reach gets staged from ladders. A good estimate says how the crew plans to get in. For the whole-unit picture, see what an ADU costs in Snohomish and King County.

A recent one: Northgate

This month we finished the siding and roofing on a detached ADU in Northgate, in north Seattle. The owner wrote this on Google afterwards, and we are quoting it because it describes the job better than we would:

“ENF did the siding and roofing on my ADU in north Seattle, specifically Northgate. He and his crew were very respectful around the property. His pricing was very reasonable and affordable when compared to his competitors. He was very patient and communicative throughout the process, and the result was more than satisfactory.”

Two things in that review are the whole point of hiring one contractor for the exterior. “Respectful around the property” is what a backyard job demands: the crew is working in someone’s garden, past their kitchen window, next to the neighbour’s fence, for two weeks. And the siding and the roof were done by the same crew under one contract, so the place where the two meet, which is where most exterior leaks start, had one person responsible for it.

Who does the exterior on an ADU

If a builder is putting up the whole unit, the exterior is part of the job. If you bought a prefab or shell, or a previous contractor left the outside unfinished, you are hiring the exterior separately, and it is worth hiring it as one package rather than a roofer, a siding crew and a gutter company who have never met. We do roofing, siding, gutters and exterior paint with one crew across Seattle, and we build ADUs in Seattle from the slab up when the project is the whole unit. Our guide on choosing an ADU builder in Washington has the questions to ask either way.

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