ADU rules in Amherst

Amherst rewrote its ADU bylaw on November 17, 2025 (effective December 1, 2025) and now runs two tracks, both of which allow a detached unit. Section 5.0111 permits a Protected Use ADU and a Local ADU in any single-family residential zoning district that allows a detached single-family dwelling by right or by special permit, naming the RO, RLD, RN, RVC and RG districts, and Section 5.0111(c) says either type may be 'detached as a separate building or within a detached structure (e.g. above an existing detached garage)', in an existing building or new construction. A detached Protected Use ADU is as-of-right on a building permit, has no minimum lot size (5.0114(a)), and gets whichever of the Article 6 dimensional standards for a detached single-family dwelling, the principal dwelling or an accessory structure is least restrictive (5.0114(b)). New detached ADUs must sit behind the front building line of the principal dwelling to the extent feasible (5.0113(j)), and no new curb cut is allowed without Public Works approval (5.0113(d)).

Section 5.0111(c) puts internal and attached units on exactly the same footing as detached ones: an ADU 'may be completely contained within the Principal Dwelling, attached to a Principal Dwelling, detached as a separate building or within a detached structure', and 'Any ADU may be within an existing building or new construction.' A basement or interior conversion that stays within the protected size is a Protected Use ADU and needs only a building permit; a larger one becomes a Local ADU. This is a real loosening: the pre-2025 bylaw allowed only a 'Supplemental Apartment' inside a single-family dwelling, capped at 800 sq ft and requiring a Zoning Board of Appeals special permit.

Section 5.0112(c) tracks the state protected-use definition: a Protected Use ADU is 'not larger in Gross Floor Area than half (1/2) the Gross Floor Area of the Principal Dwelling or nine-hundred (900) square feet, whichever is smaller.' Anything bigger is a Local ADU, which Section 5.0112(d) caps at 1,200 sq ft of gross floor area; Section 5.0115(d) excludes unfinished basement space used for storage or utilities from that figure. Gross floor area (5.0112(a)) counts basements, lofts and intermediate floored tiers of compliant ceiling height but excludes crawl spaces, garage parking areas, attics and enclosed porches, and where a lot holds several principal dwellings the largest one sets the protected cap. Height and bulk come from Article 6 and Table 3 (35 ft maximum building height in R-LD, R-O, R-N and R-VC, 40 ft in R-G); an accessory structure taller than six feet must be set back from the side lot line by a distance equal to its height (6.131), although a Protected Use ADU may claim whichever standard is least restrictive.

It depends entirely on which track you use. A Protected Use ADU carries no owner-occupancy condition: state law forbids one ('The use of land or structures for an accessory dwelling unit under this paragraph shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling', MGL c.40A s.3; 760 CMR 71.03(2)(a)), and Section 5.0114 imposes none. A Local ADU, the oversize track above the protected cap and up to 1,200 sq ft, does require it: Section 5.0115(a) says either the principal dwelling or the Local ADU 'shall be occupied by the owner(s) as their Principal Residence', made a condition of the Building Commissioner's approval and locked in by a deed restriction filed with the Registry of Deeds. Principal residence is defined at 12.41 and is determined on a preponderance of evidence (homestead declaration, tax filings, voter registration, license, and so on). Before the 2025 rewrite, owner occupancy applied to every accessory unit in town.

Section 5.0114(c) is the clean case: 'In accordance with 760 CMR, no parking space shall be required for a Protected Use ADU located within ½ mile of a commuter rail station or bus stop.' Outside that radius Amherst's general standard (7.0000, two spaces per dwelling unit unless the Permit Granting Authority sets an alternative ratio) is more than state law permits for a protected unit, which is capped at one additional space (MGL c.40A s.3; 760 CMR 71.03(2)(b)), so the state cap controls there. The local two-space default is enforceable against a Local ADU, but the Building Commissioner is the default Permit Granting Authority where the bylaw designates no board (12.36), approves the parking plan every ADU must file (5.0113(e)), and can set a lower ratio. Section 5.0113(i) separately caps provision at 'no more than two on-site parking spaces' for any ADU, and no additional or separate driveway curb cut is allowed unless Public Works approves it (5.0113(d)).

A Protected Use ADU is fully as-of-right: Section 5.0111(a) says its use 'shall be as-of-right, meeting all requirements associated with the issuance of a building permit', so a building permit from Inspection Services is the entire zoning process, with no hearing and no special permit. A Local ADU on a lot that has no ADU yet needs Administrative Approval under Section 11.212, a written Building Commissioner decision (approve, approve with conditions, or deny) filed with the Town Clerk and kept on record with Conservation and Development; the Commissioner may consult the Planning Director and apply the Section 3.204 design review criteria, but there is no public hearing. Only a second unit triggers discretion: where a lot already has a Protected Use ADU, adding a Local ADU requires a Special Permit from the Zoning Board of Appeals (5.0111(b)), which is what 760 CMR 71.03(5) and MGL c.40A s.3 require for more than one ADU.

Zoning Bylaw 5.0113(f) requires that 'If the Principal Dwelling and/or any ADU will be rented, any rental unit shall be registered and permitted in accordance with the Residential Rental Property Bylaw of the General Bylaws.' General Bylaw 3.50 (rescinded and replaced by the Town Council April 8, 2024) requires a Residential Rental Permit renewed annually by June 15, brings the unit into the town's rental inspection program, and exempts property rented fewer than 14 cumulative days a year. It defines a Short-Term Rental as 'Thirty-one (31) or fewer consecutive days' and permits it, provided the owner declares STR use on the permit application with proof of registration with the Commonwealth and payment of applicable taxes, and posts the tenant information sheet where guests can see it. Amherst has not barred short-term rental of ADUs even though MGL c.40A s.3 and 760 CMR 71.03(3)(c) would let it. On the Local ADU track the owner-occupancy deed restriction means only one of the two units on the lot can be rented out.