How Building an ADU Works in Clark County, WA: Every Step From Idea to Move-In

September 24, 2026 · Marcin Micek | Handy Pioneers · ADUs & Additions · 9 min read

Six phases, in order: feasibility, design, permits, a fixed contract, construction with inspections, and final sign-off. What happens in each, what you decide, and how long to plan for.

Building an ADU in Clark County moves through six phases: a feasibility check on your lot, design and a permit-ready plan set, permit review by the city or county, a fixed scope and contract, construction with inspections at each stage, and a final inspection before anyone moves in. For planning, most homeowners should expect roughly 6 to 12 months from first walkthrough to move-in for a conversion or attached suite, and longer for a detached unit, because construction alone can run anywhere from 3 to 12 months depending on the project [1]. The phases are the same whether you convert a garage, attach a suite, or build a backyard cottage. What changes is how much gets built.

First, what the rules now allow

Washington's ADU law requires cities and counties in urban growth areas to allow at least two ADUs on a lot, bars them from requiring the owner to live on site, and bars size caps below 1,000 square feet [2]. Vancouver allows up to two ADUs per lot at 1,000 square feet or less [3]. Unincorporated Clark County caps an ADU at 1,000 square feet, or 1,500 on lots of 20,000 square feet or more [4]. Local details still vary by address, which is why every project starts with phase one.

Phase 1: Walkthrough and feasibility (1 to 3 weeks)

Before anyone draws a line, confirm what your specific lot allows and what it will take to serve a second home. This is where the expensive surprises get caught early.

Zoning, size limits, and setbacks for your parcel
Separation from the house: in Vancouver a detached ADU must sit at least 10 feet from other structures, or add sprinklers or fire-rated construction [3]
Utilities: where water, sewer, and power come from. Vancouver asks for a Request for Utility Services, and its results, before you apply for the permit [3]
Which ADU type fits your goal and budget: conversion, attached, or detached

Phase 2: Design and drawings (4 to 10 weeks)

The idea becomes a permit-ready plan set. Clark County's residential checklist calls for a foundation plan, floor plans, cross sections, exterior elevations, framing and bracing details, engineering where needed, and Washington energy code forms including a window schedule and heating sizing [5]. This is also where most of your material decisions happen, because they are drawn into the plans: windows, exterior siding and roofing, heating type, and the finish level that sets the budget.

Phase 3: Permit application and plan review (1 to 3 months)

The plan set goes to the City of Vancouver, Clark County, or your city. Vancouver's residential process runs in five steps: application submittal, prescreen review, technical plan review, plan approval with final fees, and permit issuance, after which construction can begin [6]. State law gives local governments 65 days to issue a final decision on a complete application that needs no public notice [7]. The word that matters is complete. A thorough, well-drawn application is the single biggest thing that keeps this phase short.

Fees are part of this phase. By state law, impact fees on an ADU can be no more than half of what a new house would pay [2]. Vancouver charges ADUs 50 percent of the principal home's impact fees [3], and unincorporated Clark County waives 75 percent of its transportation, school, and park impact fees for ADUs [4].

Phase 4: Fixed scope and contract (1 to 2 weeks)

With approved plans, the price can be built from the actual drawings instead of a guess. A good contract here has a written scope, a payment schedule tied to construction milestones rather than dates, and a start date. Order long-lead items now: windows, cabinets, and anything special-order.

Phase 5: Construction and inspections (2 to 8 months, by type)

Construction follows the inspection sequence, because each stage has to pass before it gets covered up. Clark County's typical sequence runs: erosion control, setbacks and footing steel, footing drains before backfill, floor framing, shear walls, rough plumbing and mechanical, framing (with rough electrical complete), insulation, drywall nailing, and final [8]. In Vancouver, city building and electrical inspectors do the inspections [9]; outside cities that run their own electrical program, electrical permits come from Washington's Department of Labor and Industries [10].

Garage or basement conversion: roughly 2 to 4 months. The shell exists, so the work is interior: egress, insulation, systems, kitchen, and bath.
Attached suite: roughly 4 to 6 months. Foundation, framing, and a roof tie-in to the house come first.
Detached unit: roughly 4 to 8 months. A complete small house from bare ground, including utility trenching.

Phase 6: Final inspection and move-in (1 to 2 weeks)

The final inspection closes the permit. In unincorporated Clark County, an ADU needs a Certificate of Occupancy before anyone moves in [4]. Keep every document: the permit record, warranties, and a record of what was built and where. It matters for rental, insurance, and resale.

The code details that decide conversions and attached units

Converted living space in Vancouver needs a finished ceiling of at least 6 feet 8 inches and legal egress windows [3]
A garage or other unheated space that becomes living space must be brought fully up to the Washington State Energy Code [11]
Attached ADUs in Vancouver need 1-hour fire-rated construction and a sound rating of STC/IIC 45 between units [3]
Each unit needs its own lockable door, and shared utilities need separate shutoffs and independent temperature control [3]

How we run it

We handle an ADU as one accountable engagement through all six phases, with one point of contact and a written schedule. We would rather spend an extra week in feasibility than have you find out in plan review that the lot will not take the unit you designed. And once it is built, the unit joins the rest of your home in our proactive care, because an ADU that earns rent for years is an asset worth protecting.

Thinking about an ADU? Start with phase one: we will walk your lot and tell you what fits, what it takes, and what to budget. See ADU Services