ADU Cost in Portland & Vancouver WA (2026): Detached, Garage Conversion & Basement
Typical design-build ranges by ADU type, what moves the number, and how the rules differ across the river.
Quick Answer
Typical design-build ADU ranges in Portland and Vancouver, WA are $120,000–$220,000 for a garage conversion, $150,000–$260,000 for a basement ADU and $250,000–$450,000 for a detached new build. These are typical ranges, not quotes — final pricing comes after a site walk, because utility runs, foundation conditions and access vary enormously by lot. Expect 6–10 weeks of design, 8–16 weeks of permitting and 4–8 months of construction.
Masters Construction (also listed as Masters Construction Group) is a design-build general contractor serving Portland, Lake Oswego, West Linn, and Vancouver, WA.
Accessory dwelling units are the most variable project type we price. Two houses on the same street can differ by $80,000 for an identical-looking unit, entirely because of where the sewer lateral runs and whether a truck can reach the back yard. This guide gives honest ranges by type, explains what actually drives the difference, and summarizes the rules on both sides of the Columbia in plain language.
What does an ADU cost by type?
These are typical design-build ranges for completed, permitted units in the Portland and Vancouver market. Final pricing comes after a site walk.
- Garage conversion — $120,000–$220,000. The existing shell helps, but the slab usually needs insulation or replacement, the stem wall and framing must be evaluated for conditioned use, and full plumbing, electrical, insulation, egress and a new envelope are all required. Cheapest per square foot when the structure is sound.
- Basement ADU — $150,000–$260,000. Cost centers on ceiling height, egress windows (which mean cutting concrete and building window wells), moisture and vapor control, a separate entrance, and sound separation from the main house. Height is the deal-breaker to check first.
- Detached new build — $250,000–$450,000. A new foundation, full envelope, all new utility runs from the house or the street, and a complete kitchen and bath. The most expensive and the most valuable, because it is a genuinely independent dwelling with its own address-like presence.
What drives ADU cost up or down?
Utilities. The single largest swing factor on detached units. Whether water, sewer, gas and power can be extended from the existing house or require new service from the street can change the budget by $30,000 or more. Sewer depth and the location of the lateral decide whether gravity drainage works or a pump is required.
Foundation and site. Slope, soil, tree roots, and whether equipment can physically reach the build area. A backyard accessible only through a 36-inch side gate means hand-digging and wheelbarrowing concrete, and that is real labor cost.
Fire sprinklers. Required in some circumstances depending on jurisdiction, size, distance from the main structure and access. Where triggered, a residential sprinkler system plus the water service upgrade it may require is a meaningful line item — confirm applicability early, because it can change the design.
Finishes. ADUs are small, which means the fixed costs (kitchen, bath, mechanical, envelope) dominate. Upgrading finishes in 700 square feet moves the total less than people expect; changing the utility path moves it a lot.
Parking, stormwater and system development charges vary by jurisdiction and are checked as part of feasibility.
What are Portland's ADU rules, in plain English?
Portland is one of the most ADU-friendly cities in the country, and the basics have been stable for several years.
Size. ADUs are capped by a maximum floor area and by a proportion of the main house, with a commonly used ceiling around 800 square feet. Height and setback limits apply to detached units, and they are stricter near rear property lines.
Owner occupancy. Portland does not require the owner to live on site. You may rent both the main house and the ADU.
Number of units. Portland's residential infill rules allow more than one accessory unit on many lots, subject to size and configuration limits.
System development charges. Portland has waived or reduced SDCs for ADUs at various points, sometimes with conditions attached, such as a restriction on short-term rental use. The status of those waivers changes — confirm the current rule with the City before you budget around it.
Because these rules are periodically revised, treat this section as orientation rather than as authority. We verify current requirements with Portland BDS for your specific address and zone during feasibility, before you spend money on design.
What are the ADU rules in Vancouver and Clark County?
Washington state law has pushed cities toward allowing ADUs broadly, and Vancouver permits them on most single-family lots — attached, detached and garage conversions all included.
Size and placement. Vancouver applies a maximum unit size and standard setback and height limits for detached units, with design requirements intended to keep the unit subordinate to the main house.
Owner occupancy. Washington has moved away from owner-occupancy mandates for ADUs, which makes renting a Vancouver ADU straightforward in most cases.
Which jurisdiction. This is the question that trips people up: an address with a Vancouver mailing address may be in unincorporated Clark County rather than in the City of Vancouver, and the two have different codes, fees and review processes. We confirm jurisdiction before anything else.
As on the Oregon side, verify current specifics with the City of Vancouver or Clark County for your address. More on our work across the river is on the Vancouver remodeling page.
How long does an ADU take from first call to keys?
Design: 6–10 weeks. Small buildings are not simple buildings — an ADU contains everything a house contains, compressed, and the utility strategy has to be solved before the plan is fixed.
Permits: 8–16 weeks. Detached units generally take longer than conversions, and Portland typically takes longer than Clark County for equivalent scope.
Construction: 4–8 months. Garage and basement conversions sit at the low end; detached new builds at the high end, particularly where site access is constrained.
Realistically, a detached ADU started in January is a late-autumn or following-spring occupancy. Anyone promising a detached ADU in four months from first call is not counting the permit.
What should you know about renting your ADU?
Long-term rental is the common case and is permitted in both Portland and Vancouver. Design decisions that matter for a rental: a genuinely separate entrance with its own path and lighting; separate or sub-metered utilities where feasible (it prevents years of awkward conversations); sound separation from the main house, which is worth over-building in a basement or attached unit; durable finishes — LVP flooring, quartz counters, solid-core doors; and private outdoor space, even a small patio, which materially affects what the unit rents for.
Short-term rental is a separate regulatory question with its own permit requirements in both cities, and in Portland it has at times interacted with the SDC waiver. If short-term rental is the plan, confirm the rules before design, not after.
Plan for the second use case too. Many clients build for rental income and end up housing a parent, an adult child or themselves. Building in a curbless shower, a 36-inch door and blocking for future grab bars costs very little during construction and makes the unit work for both.
Design-build or a plan-set builder — which is better for an ADU?
Plan-set and prefab providers sell a designed unit and build it to a standard specification. They are efficient on flat, accessible, unconstrained lots, and their published prices are genuinely lower — as long as your site matches their assumptions.
The catch is that the site work, utility extension, foundation, permits and connection are usually excluded from the headline number and are exactly where ADU budgets are decided. A $180,000 unit on a lot needing $70,000 of utility and access work is a $250,000 project.
Design-build makes sense when the lot is sloped or hard to access, when the utility path is uncertain, when the ADU has to match an existing house architecturally, when it is a basement or garage conversion (every one of which is bespoke), or when you want one contract covering design, permitting, site work and construction with a fixed price after design.
We will tell you honestly at the consultation if your lot is a good candidate for a plan-set approach. See our ADU builder page for our own service, and our process for how the phases run.
Frequently Asked
Frequently Asked Questions
What homeowners ask us most about this work.
Typical design-build ranges are $120,000–$220,000 for a garage conversion, $150,000–$260,000 for a basement ADU and $250,000–$450,000 for a detached new build. These are typical ranges rather than quotes — utility runs, foundation conditions and site access vary so much by lot that we price ADUs after a site walk.
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