As of October 2026, a June 2026 national cost guide puts a new detached ADU at about $150,000 to $400,000 all in, or roughly $250 to $550 per sq ft. Basement and garage conversions usually cost less because the structure already exists. In Portland, waiving system development charges can remove a large fee. A site visit turns that range into a firm figure.
Which published price ranges apply to Portland?
We did not find a current published source that tracks Portland ADU construction prices on their own. These are the most relevant figures we found, as of October 2026:
- Detached ADU, national: about $150,000 to $400,000 all in; by size, about $150,000 to $250,000 for 400 to 500 sq ft and $200,000 to $325,000 for 600 to 800 sq ft (Block Renovation, June 2026).
- Attached ADU, national: roughly $100,000 to $250,000, per a 2026 Block Renovation guide.
- Garage conversion, national: about $50,000 to $150,000 before permits and fees (Block Renovation and Angi).
Smaller units cost more per square foot, because the kitchen, bathroom, foundation and sewer connection cost about the same whatever the size.
What fees can Portland add?
Block Renovation's June 2026 guide totaled Portland's fee schedule for a new detached ADU at about $36,720, including a building permit of $902, water service and connection of $10,571, sanitary sewer of $6,639, parks development of $7,101 and transportation of $3,513. Several of those lines are system development charges, which the city waives for ADUs that qualify. See the SDC waiver guide.
A Portland builder's guide from Amplified Renovations estimated building permit fees of $3,000 to $6,000 on a $200,000 to $300,000 project, and $5,000 to $15,000 for a separate water and sewer connection if the ADU needs one. Fee estimates vary with scope, so we price them for your project.
Is a basement or garage conversion cheaper than a new cottage?
Usually. A basement or garage conversion reuses walls, a floor and a roof. A detached ADU needs all of those built new, plus longer utility runs. A basement that needs headroom work or waterproofing can close the gap.
What drives the price of a Portland ADU?
- Type and size. New construction costs more than conversion; each kitchen and bathroom adds cost.
- Utilities. Electrical service upgrades and sewer or water line work can be large line items.
- Site. Slope, trees, soil and access for equipment.
- Design and soft costs. The same June 2026 guide put design and soft costs at about 10 to 20% of the project.
- Finishes. Cabinets, flooring, windows and fixtures.
What could an ADU rent for?
One Portland builder, Neil Kelly, says an ADU in Portland can usually generate $1,000 to $2,500 a month. That is a builder's published estimate, not a market study, and rent depends on size, location and finish.
When does the price depend most on your lot?
It depends on conditions you cannot see from the street. Two similar units can differ widely if one needs a new electrical service, a sewer line replacement or foundation work on a slope. Whether you take the SDC waiver also changes the fee total. That is why we visit, open the panel and trace the sewer before we give a written, itemized price.
Related
Sources, as of October 2026: Block Renovation, "Detached ADU Cost: What You'll Really Pay to Build One" (June 19, 2026); Block Renovation and Angi garage conversion and ADU cost guides; Amplified Renovations, Portland ADU guide; Neil Kelly, Portland ADU page; City of Portland ADU SDC waiver page. These are third-party estimates; your written quote replaces them.
Updated October 2026