| Max size | 1,400 sq ftor 50% of main house, whichever is less |
|---|---|
| Min size | No town minimum750 figure caps the town, not you |
| Bedrooms | Not statedno bedroom limit found |
| Type | Attached or detacheddetached: side/rear siting or board review |
| Parking | 2 spacesstate caps ADU add-on at 1 space |
| Owner-occupancy | Requiredin either the ADU or the primary home |
| Short-term rental | Prohibitedno VRBO, B&B, hotel or rooming use |
| 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 Hopkinton:
State law entitles you to convert an existing detached garage or barn into an ADU even if the structure doesn't meet today's setback rules. Hopkinton's ordinance still says the conversion is only allowed if the structure already conforms — this is one of the most common ways older garages get disqualified, and it's superseded.
State law caps the extra ADU parking requirement at ONE space and gives the homeowner — not the town — the choice of where it goes. Hopkinton's rule effectively demands two extra spaces and leaves the on-site/off-site call to the Planning Board's discretion, but that rule no longer controls.
State law says the town can't tell you where your ADU's door goes — that choice belongs to you. Hopkinton doesn't just require a separate ADU entrance; it dictates which side of the house it can't be on. That dictate is superseded.
State law says a by-right ADU shouldn't require a discretionary board hearing. If you want to put a detached ADU somewhere other than the side or rear of your house, Hopkinton requires Planning Board Site Plan Review — whether that crosses the line is a judgment call; if you site it to the side or rear as the ordinance prefers, you avoid the board entirely.
State law says Hopkinton can only hold your ADU to the same look-and-feel rules it holds a normal house to — not stricter ones. Whether requiring identical appearance, design, colors, AND materials crosses that line (versus just 'compatible' design) is a call for a land-use attorney.
Start with your rights under state law: since July 1, 2025, RSA 674:71-73 (as amended by HB 577) guarantees one ADU — attached or detached — by right wherever single-family homes are allowed, protects converting an existing detached structure even if it's setback-nonconforming, caps any extra parking requirement at one space (location the owner's choice), and bars the town from dictating your ADU's entrance location. Hopkinton's ADU ordinance (Zoning §3.12, 'Accessory Dwelling Unit') was adopted June 1, 2017 and last amended March 12, 2019 — the zoning ordinance as a whole is dated 'As Amended Through: March 10, 2020' — and no later revision to the ADU section has been identified, meaning the town has not caught up to the current state law. On the plus side, Hopkinton already permits one ADU (attached or detached, including conversions of a garage or barn, or new construction) by right via an administrative building/use permit in R-4, R-3, R-2, R-1, M-1, VR-1, VB-1, and VM-1 — and notably allows ADUs by right even in M-1/VB-1/VM-1 where single-family homes themselves need a special exception, which exceeds the state floor. Its size cap (750 sq ft floor / 1,400 sq ft ceiling, whichever is less of 1,400 or 50% of the principal dwelling) also already matches or beats the state's required range. But several provisions are superseded by the 2025 law: (1) converting an existing detached garage or barn into an ADU is only allowed if that structure ALREADY meets current setbacks — state law requires the conversion be allowed even if it's setback-nonconforming; (2) the town requires TWO extra parking spaces for an ADU (on top of two for the house) with the Planning Board — not the owner — deciding if any of it can go off-site, versus the state's cap of one extra space with owner's choice of location; (3) an attached ADU's separate entrance is barred from being on the same side of the house as the main entrance — a location dictate the state law prohibits; (4) placing a new detached ADU anywhere but the side/rear of the house triggers a discretionary Planning Board Site Plan Review, arguably the kind of board hearing a by-right ADU shouldn't require; and (5) the requirement that an ADU's architecture, colors, and materials exactly match the principal dwelling may exceed what state law allows for aesthetic standards — an attorney call. The owner-occupancy requirement and recorded deed acknowledgment, by contrast, are the kinds of rules towns may still lawfully impose. See conflicts[] for full citations. Nothing in this ordinance has been updated to reflect the 2025 state law, so where the two disagree, the state rule is the one you can rely on — confirm specifics with the Planning Department (603-746-8243) before filing.
Get a free personalized report for your address in Hopkinton — including where the 2025 law works in your favor.
Get My Free Hopkinton ADU Report →House Bill 577 (2025) amended RSA 674:72, effective July 1, 2025. Wherever Hopkinton's ordinance is stricter than this floor, the state law controls. Read the full NH ADU law explainer →
Yes — Hopkinton allows one accessory dwelling unit (attached or detached) by right in nearly all residential and mixed-use districts. 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.
1,400 sq ft — or 50% of main house, whichever is less. State law doesn't let any town cap ADU size below 750 sq ft.
Attached or detached (detached: side/rear siting or board review). Since July 1, 2025, state law allows detached ADUs by right statewide — town provisions that prohibited them are superseded.
2 spaces — state caps ADU add-on at 1 space. 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.