ADU rules in Manchester

Manchester's rewritten zoning ordinance, effective March 1, 2026, lists Accessory Dwelling Unit as an accessory use permitted by right (P) in every district that permits a single-family dwelling: R-S, R-1A, R-1B, R-2, R-M, MX-1, MX-2 and BC-1. Section 4.6.1.A.1 says an ADU may be built inside the house, attached to it, within or above an attached or detached garage, or as a standalone structure, so a detached backyard unit needs no hearing. Detached units are also treated as accessory structures under section 5.6.5.C: side or rear yard only, at least 10 feet from the side or rear lot line and 5 feet from the house, no taller than the principal structure, pitched roof, and 10 percent minimum transparency on front and street facades. No extra lot area is required; the Appendix lot standards table lists the same minimum buildable lot area for a 'Single-Family / House Including 1 ADU' as for the house alone.

The same permitted-by-right listing covers attached and internal units. Section 4.6.1.A.1 expressly allows an ADU 'within a single-family dwelling, attached to a single-family dwelling, within or above its attached or detached garage.' Only one ADU is allowed per single-family dwelling, and ADUs are allowed only as an accessory to a single-family dwelling: section 4.6.1.A.1 limits them to that use, and the building type standards bar an ADU on a lot with a Townhouse (§ 5.3.1.E.4.A) or a Cottage Commercial building (§ 5.3.1.H.4.D). An interior conversion inside a garage may not eat the parking the principal dwelling needs.

Section 4.6.1.A.4 caps every ADU at 950 square feet of gross floor area, matching the ceiling in RSA 674:72, VII. Layered on top is a proportionality rule: an ADU inside or attached to the house may not occupy an entire floor and may not exceed 40 percent of the house's gross floor area, and a detached ADU is likewise capped at 40 percent of the house's gross floor area, with the whole detached structure limited to 80 percent of the total floor area of the principal structure excluding basements. Section 4.6.1.A.4.d protects a floor: those percentages 'shall not require an ADU to be smaller than 750 square feet of gross floor area,' which tracks the statutory floor in RSA 674:72, VII. Section 4.6.1.A.3 allows no more than two bedrooms, and a detached ADU may not be taller than the house (§ 5.6.5.C.3); the House building type caps height at 2.5 stories and 35 feet.

Section 4.6.1.A.2 requires that either the principal dwelling or the ADU be owner occupied, that the owner demonstrate one of the units is the owner's principal place of residence, and that both units stay in common ownership with no transfer to condominium ownership. Enforcement is not merely on paper: before a certificate of occupancy issues for the ADU, the owner must record a deed restriction or similar document mandating owner occupancy 'in perpetuity.' New Hampshire law still permits this. RSA 674:72, VI lets a municipality require owner occupancy of one of the units (without specifying which) and adopt reasonable regulations to enforce it, and the 2025 rewrite of RSA 674:72 by HB 577 (2025, ch. 197, eff. July 1, 2025) left that paragraph in place.

Section 4.6.1.A.5 requires one additional off-street parking space for the ADU and copies the statutory flexibility from RSA 674:72, IV: the space may be provided on-site or at a legally dedicated off-site location, at the property owner's discretion. No new curb cut may be created to serve the ADU, and an ADU built inside a garage may not displace parking that the principal dwelling is required to keep. Table 7.2-A repeats the requirement as one space per ADU. Properties inside the Gaslight Historic District Overlay are exempt from the Table 7.2-A minimum parking requirements under section 6.3.5.B, though section 4.6.1.A.5 states the ADU space requirement independently, so owners there should confirm with the Planning and Community Development Department.

Table 4.5-A marks Accessory Dwelling Unit as 'P: Permitted by Right' in all eight districts where it is listed, with no conditional use permit or special exception, so review is administrative. The vehicle is a building permit: section 10.3.2.A makes it unlawful to erect, construct, convert or alter a structure or change a use without the permit issued by the Director of Planning and Community Development, and section 10.3.2.B requires a scaled plot plan. A certificate of occupancy is required before use (§ 10.3.2.D), and the owner-occupancy deed restriction must be recorded before that certificate issues. School and fire impact fees apply to the new unit under Article 11 (Table 11.5-A: $459 or $1,530 school fee depending on bedrooms and size; Table 11.5-B: $190 fire fee). No zoning board involvement is required unless a separate variance or historic-district approval applies.

Nothing in section 4.6.1.A bars renting an ADU long term, but the owner-occupancy rule means one of the two units must be the owner's principal residence, both units must stay in common ownership, and transfer of either unit to condominium ownership is prohibited, so the ADU cannot be sold off. Renting also triggers the Manchester Housing Code: section 150.100 makes it unlawful to rent or offer for rent any dwelling unit without a certificate of compliance, and the section 150.101 exemption for buildings of four or fewer units applies only where the owner's unit has been their principal residence continuously since September 2, 1986, which a newly created ADU will not satisfy. Short-term rental is regulated as its own accessory use: section 4.6.1.I defines it as transient occupancy of under 30 consecutive days and no more than 90 total days per year, requires Housing Code compliance and a valid certificate of compliance, and Table 4.5-A does not permit it in R-S, R-1A, R-1B, R-2 or R-M and allows it only by special exception from the Zoning Board in MX-1, MX-2 and BC-1. In practice that bars ADU short-term rentals across Manchester's residential neighborhoods.