ADUs are allowed in Mont VernonTypes:DetachedInternal
Types:DetachedInternal
Accessory Dwelling Unit (ADU) Rules for Mont Vernon, NH
Is your property eligible for an ADU?
# of Properties
1,134
Owner-occupied
73%
Zoning districts
5
Avg Home Value
$484,390
Median Lot Size
3.38 acres
Avg Income
$158,036
Residential Types:
1,134 housing units
Single Family
84%
Apts (4+ units)
2%
Other
15%
Zoning Districts:
1,286 parcels · 5 districts
RR
68%(875)
WR
23%(297)
R
9%(112)
LC
0%(1)
Other
0%(1)
ADU Rules
What can you build in Mont Vernon, NH?
Mont Vernon, NH follows state and local rules for Accessory Dwelling Units. Here is a concise overview of the local rules.
Eligibility
- –Internal / Attached ADU: By Right
- –Detached ADU: By Right
Interior / Attached ADU Size Limits
- –Maximum internal ADU size is the greater of 30% of the principal dwelling’s gross living area (excluding unfinished attics/basements) or 800 sq ft.
Detached ADU Size Limits
- –For a detached ADU, size is capped at 30% of the main home’s gross living area (excluding unfinished spaces), but the cap cannot be less than 800 sq ft.
Max Units
- –1 ADU per single-family lot; none permitted on duplex or multifamily lots
Owner Occupancy
- –Owner must occupy either the principal dwelling or ADU
Parking
- –No numeric minimum; requires adequate on-site parking for principal and ADU; no maximum stated; applies town-wide to residential uses.
ADU State Law
What can you build in New Hampshire?
- –New Hampshire ADU rules after HB 577 (signed July 15, 2025, eff. July 1, 2025 – Chapter 197).
- –One ADU, attached or detached, is allowed By Right on every single-family lot in any zoning district that allows single-family homes.
- –If the local ordinance is silent, the same By Right rule applies. You just need a building permit.
- –Default maximum is 950 sq ft of living space
- –Towns may set a lower maximum, but no lower than 750 sq ft,
- –Town may set a higher maximum
- –If a town specifies a size limit only for Internal or Attached ADUs and is silent on Detached units, we interpret that the state default maximum of 950 sq ft applies to Detached ADUs, since local limits apply only to the types they explicitly regulate.
- –Local zoning rules for ADUs can't be stricter than those for single-family homes, including lot size, frontage, and setbacks.
- –May require up to one additional parking space for each ADU.
- –Owner-occupancy: A town may require the owner to live on the property, but can't say which unit the owner must occupy. No requirement that ADU occupants be related to the main-house occupants.
- –Water & septic: The town can't demand a bigger system than the state DES rules require. Separate systems aren't mandatory, but you must upgrade if the existing one fails or lacks approval.
- –Conversions encouraged: Existing structures (e.g., garages, barns, etc.) may be converted even if they violate current setbacks or lot-coverage limits, and towns can't block a separate electrical panel/service for the ADU.
- –A town may bar ADUs on leased land or on lots with multiple attached single-family dwellings (e.g., townhouses).
What's allowed on your lot?
These are the municipality-wide rules. What's allowed on a specific property also depends on state laws, setbacks, floor area ratio, lot coverage, environmental constraints, topography, and other property-specific factors. Search your address on FutureLot to see what's allowed on your property.
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ADU rules in other New Hampshire municipalities
Rules change at the town line. Check the municipality your property is actually in.
Hillsborough County