Building an ADU in Portland: What It Actually Costs and Why Timing Matters in 2026

Every third conversation we have with Portland homeowners these days starts the same way: “My sister-in-law needs a place to live” or “I want rental income without selling the house.” Both roads lead to the same three letters — ADU. 

Portland made it genuinely easier to build one over the past few years. But “easier” and “simple” aren’t the same thing, and the rules have specific numbers attached that matter for your budget.

What you’re allowed to build

A detached ADU in Portland can go up to 800 square feet. That’s smaller than California’s 1,200-square-foot cap, so if you’ve been looking at ADU plans from a different state, throw those out. If you’re attaching the unit to your existing house instead of building detached, you’re capped at 75% of your main home’s floor area, up to that same 800-square-foot ceiling. 

ADUs are allowed in every residential zone in the city, and — this surprises people — you don’t have to live on the property yourself. You can rent out both the main house and the ADU if that’s the plan.

The permit fee that’s about to change

Here’s the number that actually matters for your timeline: System Development Charges on an ADU under 800 square feet have been waived, and that waiver is set to expire. Once it does, you’re looking at roughly $19,000 in SDCs on top of your regular permit and plan review fees (typically $1,900 or so). That’s a $7,000 to $12,000 swing depending on your unit size, just from filing before versus after the deadline. 

If an ADU has been on your someday list, this is the year “someday” gets expensive if you wait too long. Talk to your contractor about where the deadline actually lands and whether your project can realistically get permitted in time.

What construction actually costs

Real numbers, not the round ones you see in headlines: 

A new detached ADU built from the ground up typically runs $180,000 to $280,000, depending on finishes, foundation type, and site access. Prefab units can come in lower, roughly $100,000 to $240,000, though delivery and site prep in Portland’s older, narrower lots add cost that prefab marketing doesn’t always mention. Converting an existing garage is the budget-friendlier route at $70,000 to $160,000, assuming the structure is sound enough to build on.

Site access matters more in Portland than people expect. A lot in Laurelhurst or Irvington with alley access is a different bid than a landlocked backyard in Foster-Powell where every stick of lumber gets carried in by hand.

Will it pay for itself?

Rental math in Portland currently pencils out to roughly $1,400–$1,900 a month for a one-bedroom ADU and $1,800–$2,600 for a two-bedroom, depending on neighborhood. Run your own numbers against your specific construction cost and don’t assume top-of-market rent — vacancy and turnover eat into the return more than people budget for.

Timeline reality check

Portland’s permit review generally runs 30 to 45 days, and the city offers pre-approved standard plan sets that can shave real time off that if your design fits one of them. If you’re set on a fully custom layout, budget more review time and more back-and-forth with the plan reviewer.

Where to start

Before you fall in love with a floor plan, figure out three things: does your lot support detached or only attached, what’s your realistic construction budget once site access and finishes are factored in, and how much runway do you have before the SDC waiver changes the math. Those three answers shape everything else. 

If you want a straight read on what your lot can actually support — and where you land on that SDC deadline — get in touch with PDX Remodeling & Design. We’ll walk the property with you before you spend a dollar on plans.