AGrannio

Can You Build an ADU in Portsmouth?

Yes — Portsmouth is governed by New Hampshire's statewide ADU law, which limits what the city can restrict. A 700 sq ft detached ADU here runs about $161,500–$264,500.

Construction costs in Portsmouth track close to the New Hampshire state average.

Detached ADU (700 sq ft)

$161,500–$264,500

Garage conversion (400 sq ft)

$50,500–$92,500

Statewide protection

Yes

What applies in Portsmouth

New Hampshire's statewide standard (RSA 674:71–73) is the starting point, but parcel-level rules, fees and design review are set locally. Confirm those details with Portsmouth before design.

Estimate your Portsmouth ADU cost

Your property

ADU type

Estimated cost in New Hampshire

$161,500 – $264,500

≈ $231–$378/sq ft turnkey · midpoint $213,000

$144,900
Construction
$38,300
Design & permits
$29,800
Site work
Good news — building an ADU in New Hampshire is very likely allowed.
  • 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.
  • iOwner-occupancy varies. Owner-occupancy rules are set locally in this state.
  • ✓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: RSA 674:71–73

If you rent it out

$1,125–$1,825

est. rent/mo

8.3%

gross yield

12.9 yrs

payback

~$16,461/yr after a 7% vacancy allowance, before financing & maintenance.

Financed over 20 yrs at 7.5%, the build is about $1,716/mo — vs ~$1,475/mo in estimated rent.

Get your detailed feasibility report →

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 New Hampshire cities

ManchesterNashuaConcordDover

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.