| Max size | 750 sq ftconfirmed with the town |
|---|---|
| Min size | No town minimumbuild as small as code allows |
| Bedrooms | Up to 2confirmed with the town |
| Type | Attached or detachedneither is a simple by-right permit |
| Parking | 1 spot for ADUconfirmed with the town |
| Owner-occupancy | Requiredin either the ADU or the primary home |
| Short-term rental | Not stated |
| Utility connection | Tie into the existing electrical connection or choose a separate metersame statewide |
The July 2025 state law (RSA 674:71–73) overrides local ordinance language that conflicts with it — even language a town still has posted. Here's what that means in Portsmouth:
Since July 1, 2025, state law guarantees a compliant ADU by-right approval — no conditional-use permit or board hearing may be required. Portsmouth's ordinance still says otherwise: public notice periods, deed recordings, annual renewal, and (for many real-world ADUs) a Planning Board or ZBA hearing — well beyond the 'building permit only' floor the state now guarantees. A homeowner whose ADU meets the state's baseline rules may be entitled to skip the CUP/board-hearing track entirely, and arguably the extra abutter-notice apparatus on the Administrative Approval track too — a land-use attorney can confirm how far state law reaches into Portsmouth's specific process.
State law now guarantees that any town size cap on an ADU cannot fall below 750 sq ft. Portsmouth's simplest, staff-level approval path still steers detached ADUs toward a 600 sq ft cap — below that floor. A homeowner should be entitled to build a 750 sq ft detached ADU without needing the heavier CUP process just to reach the state's minimum.
State law caps what a town can require at ONE additional parking space, with the owner choosing on-site or off-site. Portsmouth's base rule (1 extra space) matches that, but the ordinance explicitly gives the Planning Board discretion to demand MORE parking through the CUP process — and doesn't give the homeowner the state-guaranteed choice of an off-site space. Both are likely unenforceable against a homeowner who invokes the state-law floor.
This clause is framed as an aesthetic/massing rule, not an access rule, so it may survive — but it does functionally dictate which door 'reads' as the primary entrance on the building's facade. Whether that crosses into the state's ingress/egress protection is a genuine attorney call.
Requiring an interior connecting door in every attached ADU, with no stated exception, could be read as denying a homeowner who wants a fully separate/independent unit that choice. Not as clearly a violation as a 'primary means of ingress' requirement (Portsmouth's clause doesn't say the interior door must be the primary entrance), so this is lower-confidence and needs attorney review.
This is a much more prescriptive design-matching regime than a typical single-family home addition would face elsewhere in town. Whether Portsmouth applies these same match-the-house rules to ordinary additions (making this compliant) or only singles out ADUs for this level of scrutiny (making it non-compliant) has not been confirmed — flag for attorney/staff confirmation.
It has not yet been confirmed whether Portsmouth's permitted-use table allows ADUs in every single-family district as state law now requires, or still excludes some. The §10.440 table needs to be checked directly against the city's current code before this can be marked compliant or flagged.
CONFIRMED WITH THE TOWN (August 19,2026) — ADUs allowed: 1; maximum size 750 sq ft; up to 2 bedrooms; impact fees: No; parking: 1 spot for ADU; detached-unit distance rule: Must be greater than 5' from principle structure; setbacks: Single Residence A - 30’ front, 20’side, 40’ rear. Single Residence B-30’ front, 10’ side, 30’ rear.. Town contact: Peter Britz - Director of Planning and Sustainability (603) 610-7215.
Accessory dwelling units located in the Historic District are subject to review and approval by the Historic District Commission.
Pat called city hall on August 14, 2 different departments, Planning/building inspection. 7 different contacts 3 voicemails no answer. Waiting for a call back. Pat Confirmed August 19, 2026
Start with the state law, because it now controls: since July 1, 2025, RSA 674:71-73 (as amended by HB 577) guarantees every New Hampshire homeowner one ADU — attached or detached — by right wherever single-family homes are allowed, with no special exception or conditional-use permit for that by-right unit, a size cap no lower than 750 sq ft, and at most one required additional parking space (owner's choice of on-site or off-site). Portsmouth's own ADU ordinance (Zoning Ordinance §10.814, as amended through August 7, 2023) predates that law and has not been shown to have been updated for it. The ordinance allows one ADU — attached or detached — on any lot with a single-family home, with no extra lot-area or frontage requirement beyond a normal single-family lot in the same district. But its process is built around two tracks that look nothing like the state's 'by right, building permit only' floor: an Administrative Approval track (staff-level review, but with a mandatory 30-day public notice period, certified mail to abutters, a Conditions-of-Approval letter recorded at the Rockingham Registry of Deeds, and an annually-renewed Certificate of Use) and a Conditional Use Permit track (a noticed Planning Board public hearing, sometimes preceded by a Zoning Board of Adjustment variance hearing). Size limits are 750 sq ft gross living area for both attached and detached ADUs in the ordinance text — right at the state's 750 sq ft floor — but the city's own permit flow chart shows the easiest (Administrative Approval) path for a detached ADU built into an existing accessory building tops out at only 600 sq ft, meaning a homeowner would have to go through the heavier CUP process just to reach the state-guaranteed 750 sq ft; state law overrides that, since a town cannot cap the by-right ADU below 750 sq ft. The ordinance also lets the Planning Board require MORE than the base 1 parking space through a CUP and doesn't give homeowners the state-guaranteed choice of an off-site parking space — again, state law caps the requirement at one space, owner's choice of location. It mandates an interior connecting door for every attached ADU with no stated opt-out, dictates which front door 'reads' as primary when a home has two, and imposes an unusually detailed architectural-matching regime (massing, roofline, siding, trim, foundation materials) on any new ADU construction or addition — provisions that are, at minimum, attorney-review questions under the state law's limits on entrance and aesthetic mandates. On the compliant side: owner-occupancy is required (either unit — the town doesn't dictate which, which state law allows), a two-bedroom maximum applies (state law only bars one-bedroom limits), and the two units can never be separated in ownership, including by condo conveyance (also allowed). One open question: §10.440, the table that actually assigns ADU permission and approval track by zoning district, has not been confirmed to allow ADUs in every single-family district as the state now requires — that table should be checked directly against the city's current code. Until Portsmouth's current, post-2025 code is confirmed, treat the town's extra permitting apparatus (public notice, board hearings, deed recordings, annual renewal) as likely superseded for a compliant by-right ADU, and verify with the Planning Department or a land-use attorney before relying on it.
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Get My Free Portsmouth ADU Report →House Bill 577 (2025) amended RSA 674:72, effective July 1, 2025. Wherever Portsmouth's ordinance is stricter than this floor, the state law controls. Read the full NH ADU law explainer →
Yes — and since July 1, 2025, state law guarantees Portsmouth homeowners one ADU (attached or detached) by right on any single-family lot. Portsmouth's ordinance allows one ADU too, but its process still runs through staff-level Administrative Approval or a full Planning Board Conditional Use Permit — steps state law has overridden for a compliant by-right ADU. Under RSA 674:72 (as amended effective July 1, 2025), one accessory dwelling unit — attached or detached — is allowed by right on single-family lots statewide.
750 sq ft — confirmed with the town. State law doesn't let any town cap ADU size below 750 sq ft.
Attached or detached (neither is a simple by-right permit). Since July 1, 2025, state law allows detached ADUs by right statewide — town provisions that prohibited them are superseded.
1 spot for ADU — confirmed with the town. State law caps what any town can require at one additional space, located on- or off-site at the owner's choice.
Required — in either the ADU or the primary home. Where owner-occupancy is required, state law lets the owner choose which unit to live in.