ADU rules in Northampton

A detached backyard unit is allowed by right in every residential district. Northampton's zoning has no 'accessory dwelling unit' use category: the Tables of Use and Dimensional Regulations for the RR, SR, URA, URB, URC and WSP districts list 'Two single-family dwellings on a lot' among the uses allowed by right so long as all elements of § 350-6.11 are met, and § 350-2.1 defines that use to include 'detached accessory apartments, backyard cottages, and two-unit townhouses.' In RR, SR, URA and WSP one of the two dwellings must not exceed 1,800 square feet of gross living area to stay by right; a Planning Board special permit is needed only if both dwellings are 1,800 square feet or larger. Ordinary lot dimensions still apply, and the one that bites is URA's minimum lot area of 5,000 square feet per unit, so a URA lot under 10,000 square feet cannot take a second dwelling; URB and URC need only 3,750 square feet for a single, two-family or two half-scale units, and SR, RR and WSP lot minimums are flat, not per unit.

An attached or internal second unit is likewise by right everywhere in the residential districts. Every residential table lists 'Two-family so long as all elements of § 350-6.11 are met' as allowed by right, and § 350-2.1 defines 'dwelling, two-family' as 'a residential building type in which two dwelling units are contained within a single freestanding structure, including attached accessory apartments, two-unit townhouses, and backyard cottages,' adding that 'units may be in distinct, but attached, masses.' The same section routes the term 'accessory apartment' to those two definitions, which is how a basement conversion, an internal split or an addition is classified. The City's own application addendum treats converting a single-family home into a two-family with no exterior change as the simplest of the five paths it covers. No size split applies to the attached configuration; the binding form rule is § 350-6.11C(1)'s fifty-foot maximum length of massing, plus the district envelope.

There is no ADU-specific square footage limit because the second unit is simply a second dwelling. Where two detached single-family dwellings share a lot in RR, SR, URA or WSP, one of them must not exceed 1,800 square feet of gross living area; if both are 1,800 square feet or larger the project needs a Planning Board special permit. URB and URC carry no such split. An attached two-family has no numeric cap at all, only the fifty-foot maximum mass length in § 350-6.11C(1) and the district height limit (35 feet, 50 feet in URC, 20 feet for detached accessory structures). Separately, § 350-2.1 defines a 'dwelling, half-scale' as a unit no larger than 800 square feet of gross living area, and two half-scale units count as one dwelling unit when calculating allowable density, which is how URB and URC permit up to 12 half-scale units by right.

Neither unit has to be owner occupied. No owner-occupancy condition appears on the by-right two-family or two single-family lines in any district table, nor anywhere in § 350-6.11. Northampton deliberately removed the old requirement: Ord. No. 20.172 of 3-18-2021 made uses attached to a two-family allowed by right, allowed a second detached unit on a parcel with an existing single-family home, and eliminated detached accessory dwelling units by special permit, replacing an accessory-apartment regime that had been capped at 900 square feet and required one unit to be owner occupied. State law now independently forbids the requirement: M.G.L. c. 40A § 3, as amended by the Affordable Homes Act effective February 2, 2025, provides that use of land or structures for a protected accessory dwelling unit 'shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling.' Owner occupancy survives in the ordinance only for bed-and-breakfasts and for renting a room inside an owner-occupied dwelling.

Every residential district table sets the minimum for residential uses at one space per unit, a figure reduced by Ord. No. 25.300 of 9-4-2025, and § 350-8.1C's Table of Off-Street Parking Regulations defers to the district table and otherwise sets one space per dwelling unit. Placement is regulated more tightly than count: § 350-6.11C(3) requires parking to the side or rear of a dwelling, allows no more than one space in front of a dwelling's front facade, and caps the number of cars parked beside a dwelling at four; the district tables allow at most two vehicles in the front-yard setback. The tables list a parking requirement reduction under § 350-8.10F as available through site plan approval. Massachusetts law caps a protected ADU at one additional space and bars any parking requirement within half a mile of a commuter rail station, subway station, ferry terminal or bus station, which could displace the local one-space rule near Northampton's downtown intercity bus terminal.

Administrative. Both configurations are uses allowed by right, so the approvals are a zoning permit from the Building Commissioner under § 350-4.4 (decided in writing within 30 days) and a building permit. The City's application addendum states flatly that the ordinance 'allows for two-family homes and two single-family homes on the same lot by-right, requiring only building permits and not requiring additional Planning Board permits IF they comply with zoning ordinance 350-6.11.' Planning Board site plan approval is triggered only when an applicant asks for a waiver from § 350-6.11, or, under the currently codified tables, by construction over 2,000 square feet; Ord. No. 24.261 of 12-19-2024 had already moved review of second units below that threshold to building-permit review. A special permit is required only where both detached dwellings are 1,800 square feet or larger in RR, SR, URA or WSP. Ord. No. 26.054, adopted 5-21-2026 and still uncodified, removes single and two-family homes from the site plan trigger entirely and raises the general threshold from 2,000 to 2,500 square feet.

Nothing limits who may occupy or rent either unit long-term: there is no owner-occupancy test, no family or relative restriction, and no cap on rental of the second dwelling. Short-term rental is defined in § 350-2.1 as a dwelling unit or a room within a dwelling leased for less than 28 days at a time, and every residential district table lists it as allowed by right 'only upon annual registration with the City,' valid only for the year in which registration is completed and expiring December 31. Renting a room inside an owner-occupied dwelling is an allowed accessory use for any dwelling unit and falls outside that regime. Registered operators must also register with the Massachusetts Department of Revenue, and the City Council accepted the community impact fee under M.G.L. c. 64G § 3D(a) on 4-4-2019 and § 3D(b), covering two- and three-family dwellings, on 5-2-2019. M.G.L. c. 40A § 1A expressly preserves a municipality's power to restrict or prohibit short-term rental of an accessory dwelling unit while barring unreasonable restrictions on non-short-term rental.