Can You Build an ADU in Portland?
Yes — Portland is governed by Maine's statewide ADU law, which limits what the city can restrict. A 700 sq ft detached ADU here runs about $157,000–$257,000.
Construction costs in Portland track close to the Maine state average.
Detached ADU (700 sq ft)
$157,000–$257,000
Garage conversion (400 sq ft)
$49,000–$90,000
Statewide protection
Yes
What applies in Portland
Maine's statewide standard (30-A M.R.S. §4364-B (LD 2003, 2022)) is the starting point, but parcel-level rules, fees and design review are set locally. Confirm those details with Portland before design.
Estimate your Portland ADU cost
Your property
ADU typeEstimated cost in Maine
$157,000 – $257,000
≈ $224–$367/sq ft turnkey · midpoint $207,000
- $140,700
- Construction
- $37,300
- Design & permits
- $29,000
- Site work
- iSize set locally. This state leaves ADU size limits to your city — typical caps run 600–1,200 sq ft. Check your local code.
- ✓No extra parking likely required. Set by local ordinance — commonly 1 space per ADU unless near transit.
- ✓No owner-occupancy requirement. You can rent out both the main home and the ADU — you do not have to live on the property.
- ✓Lot has room. A 700 sq ft ADU uses about 12% of your 6,000 sq ft lot — comfortably within typical coverage limits.
Rule basis: 30-A M.R.S. §4364-B (LD 2003, 2022)
If you rent it out
$1,100–$1,775
est. rent/mo
8.4%
gross yield
12.8 yrs
payback
~$16,182/yr after a 7% vacancy allowance, before financing & maintenance.
Financed over 20 yrs at 7.5%, the build is about $1,668/mo — vs ~$1,450/mo in estimated rent.
Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.
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Other Maine cities
Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.