Nashua's Accessory Structures: The Rules That Matter

Every city handles accessory structures a little differently. In Nashua, New Hampshire, there are 5 distinct rules that residents and property owners should be aware of. Some are stricter than what neighboring cities enforce, and others are more relaxed. Here is what you need to know.

Garage Conversions

Converting a Nashua garage to habitable space requires a building permit and electrical permit from the Department of Building Safety under the NH State Building Code (IRC change of occupancy from U to R-3). If the conversion adds a kitchen and separate entrance, the result is an Accessory Dwelling Unit governed by NH RSA 674:71-:73 (HB 577, 7/1/2025) — Nashua must permit the conversion by-right even where the existing garage does not meet current setbacks, and the ADU may be up to 750-950 sq ft.

Key details: Building Permit: Required (change of occupancy U to R-3). Electrical Permit: Required for added wiring. Building Code: NH State Building Code (IRC 2018 + NH amendments). ADU Trigger: Kitchen + separate entrance. ADU Minimum Size City Must Allow: 750 sq ft (RSA 674:72).

Building without permits triggers a stop-work order from the Department of Building Safety, double permit fees, and possible occupancy denial. Creating an unpermitted second dwelling unit (kitchen + separate entrance) without ADU approval is a zoning violation enforced by the Administrative Officer with civil penalties under NH RSA 676:17 of up to $275 per day first offense and $550 per day subsequent.

If you are coming from a city with tighter rules, you will find Nashua gives residents more flexibility on garage conversions.

ADU Rules

Effective July 1, 2025, NH RSA 674:71-:73 (as amended by HB 577) requires Nashua and every NH municipality to allow one accessory dwelling unit (attached or detached) by-right on every single-family lot, with no minimum size below 750 sq ft and a default maximum of 950 sq ft. Nashua's pre-2025 Sec. 190-32 capped ADUs at 30 percent of the principal dwelling or 700 sq ft, required owner occupancy, and limited rental to family — those provisions are now preempted by state law to the extent they conflict. Existing detached structures (garages, barns) may be converted regardless of current setback nonconformity.

Key details: State Statute: NH RSA 674:71-:73 (HB 577, eff. 7/1/2025). Number Allowed: 1 ADU per single-family lot (by-right). Minimum Size City Must Allow: 750 sq ft. Default Maximum Size: 950 sq ft (city may authorize larger). Attached & Detached: Both required by-right.

Local provisions that conflict with RSA 674:71-:73 (such as restricting ADU rental to family members or capping size below 750 sq ft) are preempted and unenforceable. Aggrieved applicants may appeal to the Zoning Board of Adjustment under RSA 676:5 and seek judicial review under RSA 677:4. Properly permitted ADU violations (occupancy without a building permit, unauthorized rental in violation of an owner-occupancy condition) are enforced by the Administrative Officer with civil penalties under NH RSA 676:17 of up to $275 per day first offense and $550 per day subsequent.

The rules around adu rules in Nashua lean permissive, but that does not mean anything goes.

Carport Rules

Nashua Land Use Code Sec. 190-31 (Accessory Buildings) treats unattached carports as detached accessory structures subject to a typical 4 ft side/rear setback, behind-the-front-facade rule, and the underlying district's dimensional standards in Sec. 190-16. Construction requires a building permit through the Nashua Department of Building Safety under the NH State Building Code, with engineered anchorage for the 115 mph basic wind speed in Hillsborough County. Attached carports follow the principal building's setbacks.

Key details: Code Reference: Nashua Land Use Code Sec. 190-31 + Sec. 190-16. Side / Rear Setback (detached): 4 ft typical (per accessory rules). Front Yard Carport: Prohibited (must be behind front facade). Max Height: Less than principal building (typ. 18-20 ft). Attached Carport Setbacks: Match principal building (Sec. 190-16).

Building a carport without the required building permit triggers a stop-work order from the Nashua Department of Building Safety, double permit fees, and an order to remove the structure. Setback or dimensional violations are enforced by the Administrative Officer with civil penalties under NH RSA 676:17 of up to $275 per day first offense and $550 per day subsequent.

The rules around carport rules in Nashua lean permissive, but that does not mean anything goes.

Shed Rules

Nashua does not require a building permit for a shed of 200 sq ft or less, but a Land Use Permit (zoning) is required from the Nashua Department of Building Safety regardless of shed size to confirm setback compliance under Sec. 190-16 and Sec. 190-31 (accessory buildings). A plot plan showing shed location must be submitted. Sheds in residential zones (R-40, R-30, R-18, R-9, R-A, R-B) typically must meet a 4 ft side/rear accessory setback and sit behind the principal building's front facade.

Key details: Code Reference: Nashua Land Use Code Sec. 190-31 + Sec. 190-16. Building Permit Exemption: Sheds ≤200 sq ft (no utilities). Land Use Permit: Required for any size shed. Side / Rear Setback (typical): 4 ft minimum. Front Yard Placement: Prohibited — must sit behind front facade.

Building a shed without the required Land Use Permit or exceeding the 200 sq ft building permit exemption without permits triggers a stop-work order from the Department of Building Safety, double permit fees, and an order to remove or relocate the structure if it violates setbacks. Civil penalties under NH RSA 676:17 apply: up to $275 per day first offense and $550 per day subsequent.

The rules around shed rules in Nashua lean permissive, but that does not mean anything goes.

Tiny Homes

NH RSA 674:74 defines tiny houses as small structures intended for year-round occupancy that meet the NH State Building Code and may be on a permanent foundation or chassis. Where Nashua permits detached ADUs under RSA 674:73 (it must, as of 7/1/2025), it must permit a single tiny house as a detached ADU — but a lot may have either an ADU or a tiny house, not both. Tiny houses must include facilities for sleeping, eating, cooking, and sanitation and meet all single-family building, fire, safety, and utility codes. Park-model RVs and tiny houses on wheels not meeting the State Building Code are not permitted as permanent dwellings.

Key details: State Statute: NH RSA 674:74. Building Code: NH State Building Code + IRC Appendix Q. Tiny House as Detached ADU: Required where detached ADUs allowed (RSA 674:73). ADU + Tiny House on Same Lot: Prohibited — one or the other. Required Facilities: Sleeping, eating, cooking, sanitation.

Living in a non-code-compliant tiny home or park-model RV as a permanent residence is a violation of Sec. 190-32, Sec. 190-31, and the NH State Building Code enforceable by the Nashua Administrative Officer and Department of Building Safety. Remedies include cease-and-desist orders, civil penalties under NH RSA 676:17 of up to $275 per day first offense and $550 per day subsequent, and disconnection of utilities.

The Bottom Line

Compared to many U.S. cities, Nashua gives residents more room on accessory structures. 4 of the 5 rules here are rated permissive. But permissive does not mean unregulated. There are still requirements, and the city does enforce them when violations are reported.

Keep in mind that Nashua can amend these rules at any council meeting. For the most current version of any rule mentioned here, check the specific ordinance page, where we track updates as they happen.