ADU rules in Derry

Derry rewrote its ADU section on November 18, 2025 (effective December 19, 2025) and added § 165-25L: a parcel may hold one accessory dwelling unit, 'which may be attached or detached.' The same amendment revised § 165-8, Number of Buildings Per Lot, so the one-residential-building-per-lot rule no longer applies 'in the case of an accessory dwelling unit,' which is what makes a freestanding backyard unit possible. § 165-25A separately allows conversion of existing structures, 'including but not limited to detached garages, regardless of whether such structures violate current dimensional requirements for setbacks or lot coverage.' The change tracks RSA 674:73 as amended by Laws of 2025, chapter 197, effective July 1, 2025, which flipped detached ADUs from optional to mandatory statewide: 'A municipality shall permit one detached accessory dwelling unit.' A detached unit still has to meet the district's ordinary building setbacks (§ 165-25A) and counts against the one-ADU-per-parcel limit.

§ 165-25E allows the unit to be 'contained within or be attached to the existing dwelling or be within a structure which is attached to the existing dwelling.' The December 2025 amendment softened the interior-door rule from a requirement to an option ('may include an interior door'), matching RSA 674:72, III as amended in 2025, which now says attached units shall have either independent ingress and egress or ingress and egress through shared common space and that 'the municipality shall not limit the choice of ingress and egress.' Accessory apartments are listed under 'Permitted uses' in every residential district: MFR (§ 165-44A), MHDR (§ 165-45A), MHDR-II (§ 165-45.1A) and MDR (§ 165-46A), with LMDR (§ 165-47A) and LDR (§ 165-48A) incorporating the MDR list by reference. The December 2025 Matrix of Permitted Uses marks Accessory Dwelling Unit 'P' in GC, MFR, MHDR, MHDR-II, MDR, LMDR and LDR, and 'P*' (permitted subject to district-specific restrictions) in GC IV, CBD, OBD, OMB and IND II.

§ 165-25D caps the unit: 'The total living space of the accessory dwelling area shall not exceed 950 square feet of living space.' § 165-25F adds a composition rule: the ADU 'shall be composed of a kitchen, a living room, a bathroom, and no more than two bedrooms,' and § 165-25G requires room sizes meeting the International Residential Code. The 950 figure was reset in the December 19, 2025 amendment and lands exactly on the statutory default in RSA 674:72, VII, which since July 1, 2025 reads 'The total living space of the accessory dwelling unit shall not exceed 950 square feet unless otherwise authorized by the municipality' with a floor of 750 square feet. Derry sets no separate height cap or percent-of-primary-dwelling limit for ADUs; the unit is governed by the ordinary district dimensional standards for setbacks, lot coverage and height, since RSA 674:72, I now bars extra lot size, setback, frontage or design-review requirements beyond what a single-family dwelling alone would face.

§ 165-25K is a single sentence and survived the December 2025 rewrite untouched: 'Owner occupancy must occur in either the primary or accessory dwelling unit.' It is an ongoing condition on the use rather than a one-time application test, and it does not dictate which of the two units the owner occupies. This is still lawful after the 2025 state overhaul: RSA 674:72, VI was left alone by Laws of 2025, chapter 197, and continues to say a municipality 'may require owner occupancy of one of the dwelling units, but it shall not specify which unit the owner must occupy,' and may require the owner to demonstrate that one unit is his or her principal place of residence. Derry's ordinance stops at the bare requirement: it prescribes no affidavit, deed covenant or recorded restriction, and the town's building-permit materials do not describe an owner-occupancy filing, so enforcement mechanics are worth confirming with Code Enforcement.

§ 165-25C: 'One additional off street parking space shall be provided for the accessory dwelling unit.' The requirement was reduced to a single space by the February 21, 2025 amendment, described in the ordinance's own amendment log as amending 'the setback provisions and required parking minimum' of § 165-25. One space is exactly the ceiling RSA 674:72, IV allows: since July 1, 2025 a municipality that requires parking for the principal dwelling 'may require up to one additional parking space for each accessory dwelling unit.' Two mismatches with the statute are worth knowing. First, the statute lets the owner satisfy required spaces 'either on-site or at a legally dedicated off-site location, at the property owner's discretion,' while § 165-25C says only 'off street.' Second, Derry's parking schedule in LDCR § 170-63.C applies to 'all non-residential and multi-family residential sites' and lists no minimum for a single-family detached dwelling, so the statutory predicate for charging the ADU an extra space is not obvious from the written regulations.

An ADU in Derry is administrative. § 165-25 is a pure standards section: 'The creation of an accessory dwelling unit ... shall be subject to all of the following conditions,' with no special exception, conditional use permit or planning board referral anywhere in its twelve subsections. The residential districts list accessory apartments under 'Permitted uses,' and the December 2025 Matrix of Permitted Uses, whose legend distinguishes P (Permitted), SE (Special Exception) and CUP (Conditional Use Permit), shows only P or P* for Accessory Dwelling Unit and never SE or CUP. Site plan review under § 165-16 reaches only nonresidential and multi-family development, so it does not attach. The town's Code Enforcement / Building office confirms the route in plain language: applicants file online and 'select Residential Additions & Renovations for: an accessory dwelling unit,' alongside decks and garages. All of this matches RSA 674:72, I, which since July 1, 2025 requires one ADU to be 'allowed as a matter of right' and no longer permits municipalities to route ADUs through a conditional use permit or special exception.

Nothing in § 165-25 limits tenancy length, tenant income or tenant relationship for a long-term rental, and RSA 674:72, VIII forbids Derry from requiring a family relationship between the two households. The binding limits are ownership and occupancy. § 165-25K means the owner must live in the primary or the accessory unit, so both units cannot be leased out at once, and § 165-25I bars converting 'the structure and the lot ... to a condominium or any other form of legal ownership distinct from the ownership of the existing single family dwelling,' so the ADU cannot be sold off (RSA 674:72, I likewise prohibits separate condominium conveyance unless the municipality allows it). Derry has no short-term rental ordinance, registration or licensing program: neither Chapter 165 nor the Town Code contains one, and New Hampshire has no statewide STR statute. Transient lodging is instead handled as a distinct use: a 'Tourist Home' is an owner-occupied dwelling renting to ten or fewer transient guests 'none of whom shall stay for more than fourteen days in any calendar year,' the definition expressly includes bed and breakfasts, and the December 2025 matrix permits Bed and Breakfast only in the CBD, OBD and NC districts. Because each residential district's use list is closed ('the uses permitted within this district shall be limited to the following'), nightly rental of an ADU in a residential zone is not an allowed use.