ADUs are allowed in MarlboroughTypes:DetachedInternal
Types:DetachedInternal
Accessory Dwelling Unit (ADU) Rules for Marlborough, NH
Is your property eligible for an ADU?
# of Properties
996
Owner-occupied
41%
Zoning districts
11
Avg Home Value
$180,745
Median Lot Size
2.73 acres
Avg Income
$70,148
Residential Types:
996 housing units
Single Family
66%
Apts (4+ units)
10%
Other
24%
Zoning Districts:
1,136 parcels · 11 districts
R-3 Rural Resid
38%(426)
R-2 Residential
22%(248)
R-4 Scenic Rural
13%(150)
R-1 Village Resid
13%(149)
Other
14%(163)
ADU Rules
What can you build in Marlborough, NH?
Marlborough, 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 size of 50% of the principal dwelling's living area (equivalent to one-third of combined living areas of the principal dwelling and the ADU)
Max Units
- –Single-family: 1 attached ADU per lot; detached ADUs: 0.
- –Existing commercial building (2+ yrs): 1 ADU/building by Special Exception.
Owner Occupancy
- –Owner must occupy principal dwelling or ADU as primary residence
Parking
- –2 off-street spaces/ADU; on same lot; no reductions/waivers
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.
Cheshire County