ADUs are allowed in DurhamTypes:DetachedInternal
Types:DetachedInternal
Accessory Dwelling Unit (ADU) Rules for Durham, NH
Is your property eligible for an ADU?
# of Properties
2,078
Owner-occupied
84%
Zoning districts
15
Avg Home Value
$582,148
Median Lot Size
1.10 acres
Avg Income
$140,188
Residential Types:
2,078 housing units
Single Family
96%
Other
4%
Zoning Districts:
2,732 parcels · 15 districts
RA
34%(917)
R
21%(585)
RB
19%(516)
RC
16%(427)
Other
11%(287)
ADU Rules
What can you build in Durham, NH?
Durham, 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 850 sq ft for an internal (in-home) ADU
Detached ADU Size Limits
- –Maximum floor area for a detached accessory dwelling unit is 850 sq ft.
Max Units
- –Single-family: 1 ADU per lot total (attached or detached); Duplex: 0 per lot; Multifamily: 0 per lot; JADU: none specified.
Owner Occupancy
- –No owner-occupancy requirement.
Parking
- –1 space/ADU (plus primary dwelling); max none. ADU parking may be in driveway, on-site, or off-site if legally dedicated and accessible; in Residence A and Residence B districts, no more than 3 vehicles may park on the lot regularly unless for lawful occupants, which can affect ADU parking.
Rental Restrictions
- –Short-term rentals require Special Exception and permit; must be owner's primary residence with the owner or a family member on site overnight each night.
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.
Strafford County