ADU rules in Derry

Derry allows a detached ADU. Section 165-25L, added by the November 18, 2025 rewrite that took effect December 19, 2025, says a parcel may hold one accessory dwelling unit, 'which may be attached or detached,' and the same package amended Section 165-8 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. Section 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.' Since August 31, 2026 a unit built entirely inside a preexisting detached garage or barn is a detached unit as a matter of state law (RSA 674:71, III as amended by 2026 New Hampshire Laws chapter 210), while Derry's own Section 165-5 definition still carries the superseded 2025 wording that put such a unit on the attached side; the label makes no practical difference here, because Section 165-25 applies the same twelve conditions to attached and detached units alike and the district's ordinary setbacks govern either way. The state mandate behind all of this is RSA 674:73, which since July 1, 2025 reads 'A municipality shall permit one detached accessory dwelling unit.'

Section 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,' and 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, which bars a municipality from limiting 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 Zoning Matrix of Uses and Districts, now printed inside the September 3, 2026 ordinance rather than only as a separate sheet, 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. One wrinkle: Derry's separate Section 165-5 definition of 'Attached' requires 'a common wall or habitable space between the principal dwelling unit and the ADU,' which sits awkwardly beside the same section's imported 2025 statutory sentence treating a unit inside a preexisting detached structure as attached; state law resolved the point on August 31, 2026 by calling that unit detached.

Section 165-25D caps the unit: 'The total living space of the accessory dwelling area shall not exceed 950 square feet of living space.' Section 165-25F adds a composition rule, that the ADU 'shall be composed of a kitchen, a living room, a bathroom, and no more than two bedrooms,' and Section 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 bars extra lot size, setback, frontage or design-review requirements beyond what a single-family dwelling alone would face. All three subsections read verbatim identically in the September 3, 2026 ordinance and on eCode360.

Section 165-25K is a single sentence and survived both the December 2025 rewrite and the September 3, 2026 edition 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 remains lawful: RSA 674:72, VI was left alone by Laws of 2025, chapter 197 and again by 2026 chapter 210, 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 describe no owner-occupancy filing, so enforcement mechanics are worth confirming with Code Enforcement.

Section 165-25C: 'One additional off street parking space shall be provided for the accessory dwelling unit.' The requirement was cut 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 Section 165-25. One space is exactly the ceiling state law allows, and two statutes now hold Derry there: RSA 674:72, IV lets a municipality that requires parking for the principal dwelling 'require up to one additional parking space for each accessory dwelling unit,' and RSA 674:16, VII has since September 13, 2025 barred a local legislative body from requiring 'more than one residential parking space per unit.' Effective September 13, 2026, 2026 New Hampshire Laws chapter 329 also bars a town from requiring that the space be garaged; Derry asks only for an off-street space and neither the Zoning Ordinance nor the Land Development Control Regulations demands a garage anywhere, so no local parking rule is preempted. Two mismatches with the statute are still worth knowing: RSA 674:72, IV lets the owner satisfy required spaces 'either on-site or at a legally dedicated off-site location, at the property owner's discretion' while Section 165-25C says only 'off street,' and Derry's parking schedule in LDCR Section 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. Section 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 Zoning Matrix of Uses and Districts in the September 3, 2026 ordinance, 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 Section 165-16 reaches only nonresidential and multi-family development, so it does not attach. The town's Code Enforcement and Building office confirms the route in plain language: applicants file online and 'select Residential Additions & Renovations for: an accessory dwelling unit,' alongside decks, garages and sheds. 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 Section 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: Section 165-25K means the owner must live in the primary or the accessory unit, so both units cannot be leased out at once, and Section 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. Derry has no short-term rental ordinance, registration or licensing program, and New Hampshire has no statewide STR statute. Transient lodging is instead 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 matrix permits Bed and Breakfast only in the CBD, OBD and NC districts. Each residential district's use list is closed ('the uses permitted within this district shall be limited to the following'), and the Town Administrator's August 2026 staff report on the Central Business District amendment states the interpretive rule plainly, that 'Derry has a permissive zoning ordinance, which means if the use is not listed, it is not permitted,' so nightly rental of an ADU in a residential zone is not an allowed use.