MassGIS Addressing Guidance for Accessory Dwelling Units (ADUs)

This page supplements the MassGIS Address Standard for Massachusetts Municipalities and provides specific recommendations for ADU address assignment and best practices.

Updated 3/10/25

Per 760 CMR 71.00 regarding protected use accessory dwelling units, Section 71.03 paragraph (8) states “All ADUs shall be assigned an address consistent with the most current Address Standard published by MassGIS. ADU addresses shall be reported to MassGIS and EOHLC after assignment.” This page supplements the MassGIS Address Standard for Massachusetts Municipalities and provides specific recommendations for ADU address assignment and best practices.

A detached ADU is an accessory dwelling unit that comprises a standalone building with no common walls or other structural elements shared with a principal dwelling on the same lot. An attached ADU is an accessory dwelling unit principally in new construction that shares walls or other structural elements with a principal dwelling on the same lot. Attached ADUs are considered to be an enlargement of or an addition to the principal dwelling’s building, therefore a principal dwelling and attached ADU are considered to be parts of the same single building. An internal ADU is an accessory dwelling unit that is created entirely (or almost entirely) within the preexisting building envelope of the principal dwelling, requiring minimal exterior alteration (see the EOHLC Accessory Dwelling Units page for additional information). Every detached, attached, and internal ADU should receive an address that uniquely distinguishes it from any other address in the municipality and is compliant with the best practices outlined on this page and the address standard, particularly Sections 3.11, 4.2, 4.2.2, and 4.4.

1. If an occupant of the principal dwelling or first responder can access an ADU only through a dedicated exterior entry, it is recommended that the ADU adopt the principal dwelling’s address but also append alphabetic characters as a suffix to its address number. This will be the case for all detached ADUs and any internal or attached ADUs that don’t share an interior hallway/stairwell/doorway with the principal dwelling unit. The suffix must clearly distinguish the ADU address from any other address in the municipality with the same number and street name (e.g., “19 Pine Street” is the principal dwelling address, so “19A Pine Street” is assigned as the ADU’s address, provided there is no other “19A Pine Street” or “19 Pine Street, Apt/Unit A” used as an address elsewhere in the same municipality). If the ADU’s exterior entry is located farther away from the lot’s primary egress than the principal dwelling’s main exterior entry, assigning an address number suffix of “R” (to denote “Rear”) is desirable, to facilitate public safety response. A single alphabetic character suffix is preferable, but more than one may be used if appropriate. If the principal dwelling or another property already incorporates the preferred address number suffix into its address, multiple options are available depending on pre-existing numbering along the street. Please contact MassGIS with the details for more specific guidance.
2. If a first responder or other occupant of the principal dwelling can access the ADU through an interior hallway/stairwell/doorway without having to exit the building, the ADU is functionally classified as a “unit” for addressing purposes. This type of ADU should be assigned an address that shares the same address number and street name as the principal dwelling, and an additional UNIT identifier that unambiguously distinguishes it from the principal dwelling or any other addressed units on the property (e.g. “Apartment 1”, “Unit A”, etc...). It is not necessary to also assign a separate UNIT value to the principal dwelling (e.g., the principal dwelling is (and remains) “73 Maple Avenue” and the ADU is assigned an address like “73 Maple Avenue, Apartment 1” or “73 Maple Avenue, Basement Unit”).
3. Massachusetts General Laws C. 148, S. 59 and any additional municipal guidance regarding address signage should be followed to ensure that signage is present and clearly distinguishes ADUs from the principal dwellings.

Any additional requests for clarification or guidance for anomalous addressing situations should be directed to [e-mail] with the subject “ADU Addressing Guidance”. Newly assigned ADU addresses should be reported to [e-mail] and [e-mail].

Date published:	April 30, 2025
Last updated:	May 6, 2025