Building an ADU in Portland: What It Actually Costs

Quick answer: A detached ADU in Portland typically costs $125–$250 per square foot to build, or $150,000–$300,000+ total for an 800-square-foot unit once you include design, permitting, and site work. On top of construction, budget roughly $1,900 for permitting and plan review and around $19,000 in System Development Charges (SDCs) — unless you qualify for Portland’s ADU SDC waiver, which trades that fee for a 10-year restriction on short-term rental use.

That waiver is the detail most cost guides skip, and it’s the one that actually changes your numbers. Here’s the full picture.

Portland ADU Rules That Affect Cost (2026)

Before the cost math makes sense, the zoning rules that shape it:

The lack of a parking or owner-occupancy requirement is part of why Portland has been an active ADU market — it removes two of the biggest constraints homeowners in other cities run into.

What Construction Actually Costs

$125–$250 per square foot is the working range for a new detached ADU in the Portland metro, which puts an 800-square-foot unit at roughly $150,000–$300,000+ all-in, including design and site work. Where you land in that range depends on:

• Site conditions — slope, existing utility access, and tree removal all move the number. A flat lot with utilities already stubbed nearby is cheaper than a sloped backyard requiring retaining walls and a new utility trench.
• Finish level — a builder-grade ADU sits at the bottom of the range; matching your primary home’s finishes (custom cabinetry, higher-end fixtures) pushes toward the top.
• Utility connections — running new water, sewer, and electrical service to a detached structure is one of the largest hidden costs, especially on lots where the ADU sits far from existing service lines.

Garage and basement conversions are typically less expensive than ground-up detached builds, since you’re not paying for a new foundation or roof structure — though they carry their own cost drivers: bringing an existing structure up to residential code (insulation, egress windows, ceiling height, fire separation) can eat into the savings, especially in older garages that weren’t built with living space in mind.

Permits and Fees

Two separate line items, and they’re often confused with each other:

• Permit and plan review: approximately $1,900
• System Development Charges (SDCs): approximately $19,000 — these fund sewer, water, transportation, and parks infrastructure and are assessed on new dwelling units citywide

Important nuance: Portland’s newer citywide SDC exemption for housing (active August 15, 2025 through September 30, 2028) explicitly excludes ADUs — they have their own, separate waiver program. Don’t assume the citywide exemption applies to your ADU; it doesn’t.

The ADU SDC Waiver: The Real Tradeoff

Portland’s ADU-specific SDC waiver can eliminate that roughly $19,000 SDC charge — but it comes with a binding condition:

• You sign a covenant agreeing that neither the ADU nor the primary house will operate as a short-term rental (under 30 days) for 10 years.
• The covenant is tied to the property, not the owner — it survives a sale.
• If you violate it or want out early, you owe 150% of the current SDC rate at the time of violation — not the rate you were originally waived.

For a homeowner planning to rent the ADU long-term (or house family), this is close to free money — you get the fee waived for a restriction you weren’t going to violate anyway. For a homeowner keeping optionality open for short-term rental income on either unit, it’s a real tradeoff worth running the numbers on before signing.

What an ADU Can Realistically Earn

Based on current Portland rental data:

• 1-bedroom ADU: $1,400–$1,900/month
• 2-bedroom ADU: $1,800–$2,600/month
• Estimated annual ROI: 6–9%

Run your own numbers before treating an ADU as a straightforward investment: at $200,000 in total cost and $1,700/month in rent ($20,400/year), you’re looking at roughly a 10% gross yield before accounting for vacancy, maintenance, property management, insurance, and the SDC/permit costs above. The 6–9% ROI figure already nets out more of those factors — treat it as the more realistic planning number.

Where Homeowners Get Surprised

• Utility trenching costs. If your detached ADU location is far from existing sewer, water, or electrical service, running new lines can add tens of thousands of dollars that a generic per-square-foot estimate doesn’t capture.
• The SDC waiver isn’t automatic. You have to apply for it and sign the covenant before or during permitting — it’s not something you can add after the fact if you change your mind later.
• Financing an ADU is different from financing a remodel. Many homeowners use a HELOC, cash-out refinance, or construction loan rather than straight savings — worth discussing with a lender before you’re deep into design, not after.
• Design and permitting take real time. Between design, plan review (30–45 days), and construction, most Portland ADU projects run 6–12 months from kickoff to move-in — plan the timeline accordingly if you’re counting on rental income by a specific date.

ADU Cost FAQ

Thinking about an ADU, garage conversion, or addition on your Portland-area property? PDX Remodeling & Design can walk your site, talk through what your lot and existing structures actually allow, and give you a real number instead of a per-square-foot guess. Get in touch to start the conversation.