Building an ADU in Portland: Zoning, Permits and SDCs

An accessory dwelling unit (ADU) is a second, smaller home on a lot that already has a house, attached house, duplex or manufactured home. In Portland it can be a basement or attic conversion, a garage conversion, an addition or a new detached building. The zoning rules sit in Chapter 33.205 of the Zoning Code, most recently amended by Ordinance 192213, effective 11-01-2026. Building and trade permits are issued by Portland Permitting and Development.

How many ADUs a lot can carry

• Single-dwelling zones (R20 to R2.5): one ADU with a house or manufactured home on any lot where a house is allowed. Two ADUs are allowed when the lot meets the minimum lot area below, and at least one of the two must be detached.

• RF zone: one ADU.

• Street frontage: two ADUs are not allowed on a lot without frontage on a maintained street, unless the lot fronts a private street that connects to one. A payment in lieu of street improvements does not count.

• Duplex: one ADU, detached from the duplex, on a lot that meets the same minimum. It is not allowed where the duplex is two detached units.

• Multi-dwelling, commercial, CI1 and EX zones: up to two ADUs with a house, attached house or manufactured home, and one with a duplex.

• Not allowed: with a triplex, fourplex, cottage cluster or multi-dwelling structure.

Minimum lot area for two ADUs, or a duplex with an ADU (Table 205-2): 12,000 sq ft in R20, 6,000 in R10, 4,200 in R7, 3,000 in R5 and 1,500 in R2.5.

Size, height and placement

• Size: no more than 800 sq ft of living area, or 75 percent of the main dwelling’s living area, whichever is less. A 1,000 sq ft house allows a 750 sq ft ADU. The cap does not apply to a basement conversion when the house has been on the site at least 5 years.

• Height: a detached ADU can reach 20 ft outside the required setbacks and 15 ft inside them. Side and rear setbacks are often 5 ft, depending on the zone.

• Location: a detached ADU must sit at least 40 ft from the front lot line or behind the rear wall of the house.

• Coverage: a detached ADU cannot cover more ground than the house, and all detached accessory structures together are capped at 20 percent of the site.

• Design: a detached ADU taller than 15 ft must match the house in finish materials, roof pitch, trim, eaves and window orientation, or meet the City’s listed alternatives.

• Visitability: with two ADUs, or a duplex plus an ADU, one unit needs a no-step entrance, a bathroom with a 60-inch turning circle, 200 sq ft of living area on the entrance floor and 34-inch doors. Lots averaging a 20 percent slope or more, and conversions of structures at least 5 years old are exempt.

• Parking and occupancy: no on-site parking is required. Oregon law bars cities from requiring off-street parking or owner occupancy for an ADU.

A design that misses a standard needs an Adjustment, a discretionary review the City puts at about 8 to 10 weeks, or roughly 40 to 50 business days, with no guaranteed outcome.

Building code items that drive cost

An ADU inside or attached to the house is built to the Oregon Residential Specialty Code as a two-family dwelling, as modified by the City’s Building Code Guide 00-10.

• Separation: walls and floors between the units need a one-hour fire-resistance rating, an STC 45 sound rating and, for floor and ceiling assemblies, an IIC 45 impact rating.

• Systems: each unit needs its own independent heating and cooling. ADU circuits can share no loads with the house, and a shared panel must sit in a common area both units can reach.

• Headroom: 6 ft 8 in for hallways and the ADU entrance door, and 6 ft 4 in at stairs. Ceiling height, egress windows and stairs are where conversions get expensive or fail.

• Slopes: a slope of 20 percent or more, or a landslide hazard area, requires a geotechnical report.

• Trees: trees with trunks over 12 inches need protection, inspected before work continues.

• Electrical: ADU electrical work must be done by a licensed electrical contractor, even when the owner does the rest.

Water, sewer and stormwater

• The ADU can share the house’s water meter. A separate meter adds Water Bureau installation fees.

• A shared sewer connection is an option, with conditions. A new connection to the public sewer needs a sewer lateral permit.

• New fixtures set lower than the uphill manhole cover need a backwater valve.

• Rainwater generally stays on the site, and 500 sq ft or more of new impervious surface triggers a stormwater facility.

• An abandoned septic tank within 10 ft of new footings must be located and decommissioned.

System development charges

System development charges (SDCs) pay for transportation, sewer, parks and water capacity, and the City warns they can add significant cost to an ADU. Portland waives them on a new ADU when the owner signs a covenant that neither the ADU nor the house will be rented as a short-term rental for 10 years from final inspection. Using any part of the property as a short-term rental, or revoking the covenant, costs 150 percent of the current SDCs.

The waiver application goes in by email with the permit number, and approval is part of permit review, so file it early. The separate citywide SDC exemption for new housing, which covers permits issued from 08-15-2025 through 09-30-2028, excludes ADUs.

The permit steps

1. Research the lot on PortlandMaps: zone, permit history, and slope and flood hazards. Unpermitted work already on the site must be permitted as if it were new.

2. Book a free 15-minute appointment with a planner, building reviewer or utility expert for any question that could change the design.

3. Prepare the building permit application, site plan and plans. Conversions with structural changes need structural plans. The City’s four pre-approved detached ADU plans can speed review where they fit the lot.

4. Apply on Development Hub PDX (DevHub) through the Single PDF process. Electrical, plumbing and mechanical permits can share the building permit number or follow later.

5. Answer the correction checksheet electronically. The permit is issued once all fees are paid.

6. Schedule inspections through DevHub, including erosion control and tree protection where they apply, through final approval.

How long it takes

Portland Permitting and Development publishes a permit review dashboard, updated every Monday, that tracks business days from submittal to “Under Review” and time to approval. The City says plan complexity drives review time. The fastest path is a complete first submittal that answers zoning, utilities and trees before a reviewer has to ask.

Common questions

Can I build two ADUs on my Portland lot?

Yes, in the R20 to R2.5 zones, when the lot meets the Table 205-2 minimum (3,000 sq ft in R5) and fronts a maintained street. At least one of the two must be detached.

How big can a Portland ADU be?

800 sq ft of living area or 75 percent of the house’s living area, whichever is less. A basement conversion in a house at least 5 years old is exempt from the cap.

Do I need parking, or do I have to live on the property?

No to both. Portland requires no on-site parking for an ADU, and Oregon law bars cities from requiring owner occupancy.

Can I avoid system development charges?

Yes, through the City’s ADU SDC waiver, if you sign a covenant not to use the property as a short-term rental for 10 years after final inspection.

How SASHKOV handles it

• Checks the lot first, in writing: zone, lot area, street frontage, slope, trees and utilities.

• Tests whether a City pre-approved plan fits the lot, or prepares a site-specific permit set.

• Brings in the surveyor, arborist, geotechnical or structural engineer the project needs, under one contract.

• Files on DevHub, answers corrections and coordinates inspections through final approval.

Where a building requires a licensed architect or engineer of record, SASHKOV brings them into the contract.

More on how SASHKOV prepares the set: Permit and Construction Documents. For duplexes, cottage clusters and lot splits, see Residential and Middle Housing and the Portland middle housing guide.

Planning an ADU in Portland? Send the address and what you want to build, and SASHKOV will confirm what the lot allows and map the permit path.