What's the cheapest type of ADU to build?

A garage conversion, because it reuses the existing foundation, walls, and roof. You're mainly paying to add insulation, utilities, egress, and finishes rather than building from the ground up.

How big can an ADU be in Clackamas County?

In most cases up to 900 square feet, though your specific zone and lot can affect the limit. That cap also sets a practical ceiling on total cost.

Guide

How Much Does an ADU Cost in Clackamas County?

Detached, attached, prefab, or custom: here's what an accessory dwelling unit actually costs to build in Clackamas County, with per-square-foot pricing and rental-income math.

ADU costs in Clackamas County depend heavily on type and size. A garage conversion is the most affordable path; a detached, ground-up cottage is the most expensive. Here's the real math, including per-square-foot pricing and rental-income considerations.

Cost by ADU type

Garage conversion: the budget path: reuses existing structure; often $100,000–$180,000
Attached ADU: mid-range, sharing a wall and some systems with the main home
Detached new-build: the most expensive: new foundation, structure, and utilities; often $200,000–$350,000+
Prefab/modular detached: can trim cost versus fully custom, with less design flexibility

Per square foot

Detached ADUs commonly run in the $250–$400 per square foot range once you account for a new foundation, full utilities, a kitchen, and a bath packed into a small building. Conversions come in lower because the shell already exists. Since most ADUs here are capped around 900 square feet, size sets an upper bound on total cost.

The rental-income math

Many owners build to rent. The key numbers are your all-in build cost, the local monthly rent a unit that size commands, and your financing. A detached unit costs more but usually rents for more and adds more resale value; a garage conversion has a lower entry point and can pay back faster. Run those numbers before you pick a type.

What else affects the price

How far water, sewer, and power have to run to the unit
Site conditions: slope, access, and soils on hillside or rural lots
Finish level, and whether you go custom or prefab
Permitting and system-development charges

Frequently asked questions

What's the cheapest type of ADU to build?: A garage conversion, because it reuses the existing foundation, walls, and roof. You're mainly paying to add insulation, utilities, egress, and finishes rather than building from the ground up.
How big can an ADU be in Clackamas County?: In most cases up to 900 square feet, though your specific zone and lot can affect the limit. That cap also sets a practical ceiling on total cost.