How much does it cost to build an ADU in Portland?

A detached, new-build ADU in Portland generally runs $200,000–$400,000, about $250–$400 per square foot. A garage conversion is cheaper, around $90,000–$175,000, and a basement conversion can start near $80,000. Site conditions, utility runs, and finish level drive the spread — and Portland sits above the national average.

Is a detached ADU or a garage conversion cheaper?

A garage conversion is almost always cheaper per project because it reuses an existing foundation, walls, and roof. A detached ADU costs more but gives you a purpose-built, full-value dwelling with better light, privacy, and rental appeal — which usually returns more in both rent and resale. The right choice depends on your lot and your goal.

Why are ADUs so expensive to build in Portland?

An ADU is a complete small house — foundation, framing, roof, kitchen, bath, and all systems — so it carries most of a home's fixed costs on a small footprint, which raises the per-square-foot number. Portland's above-average construction labor, utility connection costs, and permitting add to it. Garage and basement conversions avoid some of these fixed costs.

Do ADUs pay for themselves in Portland?

Often, over time. A well-built Portland ADU can rent for roughly $1,500–$2,500 a month depending on size and location, and it adds measurable resale value. Whether it pencils out depends on build cost, financing, and how long you hold the property — run the rent-and-resale math before you commit, not after.

How long does it take to build an ADU in Portland?

Plan on roughly 5–9 months of construction for a detached ADU after design and permitting, and somewhat less for a conversion. Permitting can add significant time, so a contractor in the city's FIR program can be a real schedule advantage on qualifying accessory structures.

Guide · Costs

What a Portland ADU really costs

Detached vs. conversion, a line-item breakdown, the fees people forget, and the rent-and-resale math that decides whether an ADU is worth building.

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CostsBy The Portland Remodel Review, The Editorial TeamPublished June 16, 2026

An ADU is one of the best uses of a Portland lot: rental income, a place for family, room to flex later. It's also the project people most often under-budget, because they price it like a big remodel when it behaves like building a (very small) house. So here are real Portland ADU cost ranges for 2026, the line items that get missed, and the math that tells you whether it pencils.

How we arrived at these numbers: we cross-reference national ADU cost guides (see Sources below) and regional construction-cost indices, then adjust for Portland's roughly 10% cost premium and higher local ADU pricing. They are planning ranges, not quotes, and they exclude land. Fees and program rules change — confirm current permitting and system charges with the City of Portland. Last updated June 2026.

The four ways to build, and what each costs

“ADU” covers four pretty different projects, and the cost gap between them is large. The cheapest reuses structure you already have; the most expensive builds a new small home from the ground up.

Portland ADU cost by type (2026, excludes land)

ADU type	Typical cost	Notes
Basement / internal conversion	$80k–$175k	Reuses the existing shell; ceiling height and egress are the wild cards
Garage conversion	$90k–$175k	Reuses foundation, walls, roof; cheapest path to a separate unit
Attached ADU (addition)	$150k–$300k	New build tied to the house; shares one wall and some systems
Detached new construction	$200k–$400k	A purpose-built small home; best light, privacy, and rental value

Roughly $250–$400 per square foot for detached construction. Difficult sites, high-end finishes, and long utility runs push toward the top of these ranges; Portland generally runs above the national average.

For the full picture of design and construction options, see our Portland ADU construction guide. The right type is a function of your lot, your budget, and whether you are optimizing for lowest cost or highest long-term value.

Where the money goes on a detached ADU

A detached ADU's budget looks different from a remodel because so much of it is the fixed cost of making a building exist — foundation, shell, and systems — spread over a small area.

Where the budget goes on a detached Portland ADU

Category	Share of budget
Foundation, framing & shell	30–35%
Interior finishes & cabinetry	18–24%
Plumbing, electrical & HVAC	15–20%
Site work & utility connections	8–14%
Kitchen & bath fixtures/appliances	6–10%
Design, permits & system charges	8–12%

Conversions shift this mix sharply — they spend far less on shell and site work and proportionally more on systems and finishes.

The costs people forget

The build cost is the headline; these are the line items that surprise people:

Utility connections. A detached ADU may need new or upsized water, sewer, and electrical service. Long runs across a lot get expensive fast.
System development charges (SDCs) and permit fees. Portland has at times offered relief on these for ADUs, but programs change — confirm current charges with the city before you budget, because they can swing the total meaningfully.
Site conditions. Slope, soil, tree-root and stormwater requirements, and access for equipment all add cost on Portland's varied lots — from the West Hills grades to flatter eastside lots.
Design and engineering. A small building still needs real drawings and, often, structural engineering. Budget for it rather than treating it as overhead.

The permit timeline — and the FIR advantage

Accessory structures go through city permitting, and standard review can add months to an ADU schedule. A contractor enrolled in Portland's FIR (Field Issuance Remodel) program can permit and inspect qualifying accessory structures in the field, which compresses the timeline and the carrying costs that come with it. Since enrollment is now closed to new firms, it is worth asking whether yours has it.

Does it pencil out? The rent-and-resale math

Whether an ADU is “worth it” comes down to three numbers: what it costs to build, what it earns or saves, and how long you hold the property. A Portland ADU can rent for roughly $1,500–$2,500 a month depending on size, finish, and neighborhood, and a well-built unit adds resale value beyond its rental income. Run a simple model before you commit: build cost and financing on one side; monthly rent (or the avoided cost of housing family) plus expected resale lift on the other. A garage conversion often has the fastest payback; a detached unit usually has the higher ceiling on long-term value.

Paying for it

ADUs are commonly financed with a HELOC, a cash-out refinance, or a renovation/construction loan that lends against the property's post-build value. Because an ADU produces income, some owners also weigh the projected rent in their financing plan. Talk to a lender early — financing can sit on the critical path next to design and permitting.

How long an ADU takes to build

An ADU is a small building, but it still moves through every phase a house does. Here's the typical timeline; for the detail behind each phase, see our remodel process & timeline guide.

Typical Portland ADU timeline

Design & feasibility

1–3 months

Confirm what your lot allows, then full drawings and selections.

Permitting

1–3 months

Accessory-structure review; a FIR-enrolled firm can compress qualifying work.

Construction

5–9 months

Foundation and shell (or the conversion), systems, and finishes.

Punch list & closeout

1–2 weeks

Final touch-ups, walkthrough, and certificate of occupancy.

Who we recommend

When you are ready to scope an ADU against your specific lot, our pick is LUX Construction — a licensed Portland design-build firm that handles ADUs end to end, from feasibility and design through permitting and construction, and an enrolled FIR participant. They can tell you quickly whether your lot favors a conversion or a detached build, and what it will realistically cost. Reach them through our contact page, or compare our broader take on the best Portland remodeling firms.

Sources

HomeGuide — How Much Does It Cost to Build an ADU? — Detached $150k–$285k, garage conversion $80k–$150k, attached $150k–$300k; $150–$300/sq ft nationally
Angi — How Much Does an ADU Cost to Build? — National ADU average ~$180,000; most spend $40,000–$360,000
Relative Construction Costs by State (cost index) — Portland construction index ~1.10 — about 10% above the U.S. average, lifting local ADU costs above these national figures

Frequently asked questions

How much does it cost to build an ADU in Portland?: A detached, new-build ADU in Portland generally runs $200,000–$400,000, about $250–$400 per square foot. A garage conversion is cheaper, around $90,000–$175,000, and a basement conversion can start near $80,000. Site conditions, utility runs, and finish level drive the spread — and Portland sits above the national average.
Is a detached ADU or a garage conversion cheaper?: A garage conversion is almost always cheaper per project because it reuses an existing foundation, walls, and roof. A detached ADU costs more but gives you a purpose-built, full-value dwelling with better light, privacy, and rental appeal — which usually returns more in both rent and resale. The right choice depends on your lot and your goal.
Why are ADUs so expensive to build in Portland?: An ADU is a complete small house — foundation, framing, roof, kitchen, bath, and all systems — so it carries most of a home's fixed costs on a small footprint, which raises the per-square-foot number. Portland's above-average construction labor, utility connection costs, and permitting add to it. Garage and basement conversions avoid some of these fixed costs.
Do ADUs pay for themselves in Portland?: Often, over time. A well-built Portland ADU can rent for roughly $1,500–$2,500 a month depending on size and location, and it adds measurable resale value. Whether it pencils out depends on build cost, financing, and how long you hold the property — run the rent-and-resale math before you commit, not after.
How long does it take to build an ADU in Portland?: Plan on roughly 5–9 months of construction for a detached ADU after design and permitting, and somewhat less for a conversion. Permitting can add significant time, so a contractor in the city's FIR program can be a real schedule advantage on qualifying accessory structures.

Thinking about an ADU?

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