ADUs are allowed in LondonderryTypes:DetachedInternal
Types:DetachedInternal
Accessory Dwelling Unit (ADU) Rules for Londonderry, NH
Is your property eligible for an ADU?
# of Properties
9,443
Owner-occupied
85%
Zoning districts
16
Avg Home Value
$461,867
Median Lot Size
1.00 acres
Avg Income
$131,282
Residential Types:
9,443 housing units
Single Family
95%
Apts (4+ units)
2%
Condos
1%
Other
2%
Zoning Districts:
10,719 parcels · 16 districts
AR-I
86%(9,241)
R-III
5%(507)
IND-II
3%(283)
C-I
2%(265)
Other
4%(423)
ADU Rules
What can you build in Londonderry, NH?
Londonderry, 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
Size Limits
- –Maximum size of 40% of the living area (habitable area) of the principal dwelling
Max Units
- –1 ADU per lot for owner-occupied single-family dwellings.
- –Duplex and multifamily ADUs not permitted.
- –1 total ADU per lot regardless of ADU type (detached, attached/internal, or free-standing).
Owner Occupancy
- –Owner must occupy principal or ADU.
Parking
- –1 space/ADU; must be in rear or side yard; no additional curb cut
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.
Rockingham County