Building a Detached ADU: The Construction Sequence, Inspections, and Materials

September 24, 2026 · Marcin Micek | Handy Pioneers · ADUs & Additions · 8 min read

A detached ADU is a small house built from bare ground. Here is the build in order, every inspection that sets the pace, and the material choices that matter most in our climate.

A detached ADU is built in the same order as any new house: site work and utilities, foundation, framing and roof, rough plumbing, mechanical, and electrical, insulation, drywall, finishes, and a final inspection. Each stage has an inspection that must pass before the work gets covered, and that sequence sets the pace of the whole build [1]. For planning, the construction phase of a detached unit typically runs about 4 to 8 months once the permit is issued. Here is what happens in each stage, and the material decisions that matter most for a small home in the Pacific Northwest.

Before the build: siting and the permit

Where the unit sits drives cost as much as its size. In Vancouver a detached ADU has to be at least 10 feet from other structures, or add sprinklers or fire-rated construction [2], and ADUs in both Vancouver and unincorporated Clark County are generally capped at 1,000 square feet [2][3]. Distance from the street and the main house also sets how far water, sewer, and power have to be trenched. For the full design and permit side, see our step-by-step ADU process guide.

Stage 1: Site work and foundation (about 3 to 6 weeks)

Erosion control goes in first and is itself inspected. Then excavation, utility trenching, footings, and the foundation. Inspectors check the setbacks and footing steel before concrete is poured, and footing drains and stormwater work before backfill [1].

Material choice: slab-on-grade or crawlspace. A slab keeps the unit low to the ground, which suits a single-level, accessible design. A crawlspace raises the floor and leaves access to plumbing and ducts underneath, and takes more material and more steps on the site. On a sloped lot the decision often makes itself.

Stage 2: Framing, roof, and dry-in (about 3 to 6 weeks)

Floor framing, walls, and roof go up, then sheathing, roofing, windows, and doors until the unit is weather-tight. Floor framing and shear walls (the braced walls that resist wind and earthquakes) are inspected along the way [1]. In our climate the goal is to reach dry-in fast, because every week the frame is open to rain is a week of risk to everything that follows.

Roofing: architectural composition shingles are the common choice; standing-seam metal costs more up front and lasts longer
Siding: fiber cement lap siding is popular in the Northwest because it does not rot the way wood can; real wood looks great and needs more upkeep
Windows: they have to meet the energy code values on the window schedule submitted with the permit [4], so choose them during design, not after

Stage 3: Rough-ins (about 2 to 4 weeks)

Rough plumbing and mechanical are inspected, and rough electrical must be complete before the framing inspection [1]. Decide now where every outlet, light, and appliance goes, because changing them after drywall is expensive.

Material choice: heating. Washington's current residential energy code no longer mandates heat pumps for space heating [5], but new homes have to earn energy credits [5], and efficient heating is one of the common ways to earn them. For a small unit, a ductless heat pump is a popular choice because it heats and cools efficiently, and Vancouver requires each unit to have its own temperature control anyway [2].

Stage 4: Insulation and drywall (about 2 to 4 weeks)

Insulation is inspected before it is covered [1]. A small home with a well-sealed, well-insulated shell costs little to heat for decades, so this is not the place to cut. Then drywall, and the drywall nailing inspection.

Stage 5: Finishes and exterior (about 4 to 8 weeks)

Cabinets, counters, flooring, tile, trim, doors, paint, and fixtures inside; siding details, trim, gutters, and final grading outside. In a rental unit, durable finishes pay for themselves: luxury vinyl plank or tile over carpet in wet areas, quartz or solid-surface counters, and semi-gloss paint on trim.

Late finish decisions are the most common cause of delay at this stage. Make selections during design and confirm them before construction starts.

Stage 6: Final inspection and occupancy (about 1 to 2 weeks)

The final inspection closes the permit, and in unincorporated Clark County an ADU needs a Certificate of Occupancy before anyone moves in [3]. Get copies of everything: the permit record, warranties, appliance manuals, and a record of what is behind the walls.

How we build it

We run a detached ADU as one accountable project with one point of contact, a written schedule, and a walkthrough at each milestone. And because a rental unit is an asset for decades, we build it to be maintained, then keep it maintained through the 360 Method.

Planning a backyard ADU? We will walk your lot, map the utilities, and give you a realistic scope before design begins. See Detached ADUs