Building an ADU is one of the smartest investments Portland-area homeowners can make, but understanding the true costs is essential for planning. Based on current market data and our experience with hundreds of projects, here's a comprehensive cost breakdown.
Average Cost by ADU Type
Garage Conversion
$120,000 – $180,000~500 sq ft • $240-$360/sq ft
Attached ADU
$150,000 – $250,000~550 sq ft • $275-$450/sq ft
Detached New Build
$180,000 – $350,000+~675 sq ft • $300-$500/sq ft
Basement Conversion
$140,000 – $250,000~675 sq ft • $210-$370/sq ft
Where Does the Money Go?
Understanding the cost breakdown helps you make informed decisions about where to invest and where to save:
Architectural plans, engineering, permit fees ($6k-$10k)
System Development Charges – can be waived in Portland
Excavation, foundation, utility connections
Framing, roofing, siding, windows, insulation, HVAC
Flooring, cabinets, countertops, fixtures, appliances
The ROI of an ADU
Despite the significant investment, ADUs offer excellent returns in the Portland market. A well-designed ADU can generate $1,200-$2,500+ per month in rental income. Property value typically increases by 20-30% of the ADU construction cost. Many homeowners see full payback within 7-10 years through rental income alone.
Get Your Personalized ADU Estimate
Every project is unique. Tell us about yours for an accurate, no-obligation estimate.