Boston ADU Conversions: Fire Sprinkler + Fire Alarm Requirements When Finishing Basements and Attics
- 3 days ago
- 7 min read
Accessory Dwelling Units (ADUs) are a common way for Boston-area property owners to create additional housing—often by finishing existing unfinished space like a basement or attic. From a fire/life-safety standpoint, these projects are less about “ADU” as a label and more about what the building becomes after the work: a single-family with an internal ADU, a two-family becoming three units, or a three-family becoming four units.
This article focuses only on ADUs created by converting existing unfinished space inside the building envelope. It does not cover detached ADUs or building additions. The discussion summarizes typical Boston expectations for:
Automatic sprinklers (where required, and which NFPA standard typically applies)
Smoke/CO detection vs. fire alarm detection
When a fire alarm control unit (FACU) / manual fire alarm system is used for sprinkler monitoring and occupant notification
The practical impact of the 520 Hz low-frequency sleeping-room requirement in Group R-2 projects
This guidance reflects our coordination and code interpretation discussions with the Boston Fire Department, including Fire Protection Engineer Chris Nelson, and the applicable Massachusetts code framework.
Important: Requirements can vary by building layout, egress configuration, construction scope, and the code path accepted by the Authority Having Jurisdiction (AHJ). Treat this as an educational overview—not a substitute for project-specific review.
Scenario 1: Single-family home converting unfinished basement or attic space into an ADU
When an ADU is created from conversion of unfinished space inside a single-family home, the sprinkler and fire alarm scope is typically straightforward. In the Massachusetts residential code framework, a single-family + ADU configuration typically does not require a fire sprinkler system or a fire alarm system solely because an ADU is added. Instead, the life-safety focus is on 120V smoke and CO alarm compliance. The typical expectation is that the interior stair serving both the existing home and the new ADU is protected by interconnected smoke/CO alarms, with a smoke/CO alarm provided at each level of the stair. The intent is to ensure that if a fire occurs in either portion of the home, occupants in the other portion receive prompt warning through interconnected alarm coverage on the shared egress path.
Scenario 2: Existing two-family (two-story), convert unfinished basement into an ADU
For an existing two-family building that converts an unfinished basement into an ADU, a fire sprinkler system is required in the work area (i.e., the ADU). Because the project results in three dwelling units (R-2 occupancy), the sprinkler system may be designed as an NFPA 13D system under this framework. The owner may select one of two water supply approaches: (1) a pump-and-tank arrangement (20 minutes of water supply), or (2) a dedicated fire service line supplying the sprinkler system.
On the fire alarm side, a building fire alarm system is not required when the residential floors are two stories above the lowest level of exit discharge (780 CMR Section 907.2.9.1). The baseline intent is to provide targeted detection/notification features to support the added unit and the common means of egress. The building is required to provide 120V interconnected smoke/CO alarms within the common interior stair serving all three units (typically one at each level of the stair). In addition, a 120V heat alarm is to be installed within 6 feet of the ADU entry door inside the ADU, and that heat alarm is to interconnect with the common-stair smoke/CO alarms. Importantly, the common-stair smoke/CO alarms and the ADU entry-side heat alarm do not need to interconnect with the in-unit smoke/CO alarms within the ADU or within the existing dwelling units. This separation provides building-wide warning via the stair alarms when the ADU entry-side heat alarm activates, while avoiding nuisance interconnections between in-unit devices.
Sprinkler monitoring and exterior notification depend on the water supply. If the NFPA 13D sprinkler system is supplied by a pump-and-tank, a fire alarm control unit for sprinkler monitoring is not required. If the system is supplied by city water, then a fire alarm control unit is required to monitor the sprinkler system. In either case where sprinklers are provided, a red lens beacon and bell on the exterior wall facing the street is to be installed and arranged to activate upon sprinkler waterflow.
Scenario 3: Existing two-family (three-story), convert unfinished basement into an ADU
For a three-story building that is currently a two-family, converting an unfinished basement into an ADU results in a three-unit configuration (R-2 occupancy) and similarly requires a sprinkler system within the ADU. In this scenario, the sprinkler system is designed in accordance with NFPA 13D within the ADU, with either (1) a pump-and-tank arrangement (20 minutes of water supply) or (2) a dedicated fire service line.
Unlike the two-story two-family case, this scenario adds a stronger fire alarm component requirement. On the fire alarm side, a manual fire alarm system is to be provided to monitor the sprinkler system and to activate notification devices within the ADU (780 CMR 34 Section 904.2), as the residential floor becomes three stories above the lowest level of exit discharge (780 CMR Section 907.2.9.1; and 780 CMR 34 Section 1011.6). Heat detectors with sounder bases are to be installed within 6 feet of the entry door inside the non-ADU dwelling units, and low-voltage fire alarm smoke detectors are to be installed at each level of the common stairs connecting the units. The intent is that the common stairs—the primary shared egress element—receive reliable detection coverage, while entry-adjacent heat detection and/or sprinkler activation provides an additional layer of warning/activation without relying on in-unit smoke/CO alarms to drive building-wide response.
Scenario 4: Existing two-story two-family, convert unfinished attic (third floor) into an ADU
When an unfinished attic is converted into an ADU in a two-story two-family building, the project changes the building into a three-story, three-unit configuration. In this case, the sprinkler scope and standard increase: a sprinkler system designed per NFPA 13R is to be provided within the attic ADU space and also within the affected common stairs serving the ADU, reflecting increased life-safety expectations associated with the additional story and the change to a three-unit R-2 configuration (780 CMR Table 504.4; and 780 CMR 34 Section 1011.6).
On the fire alarm side, a manual fire alarm system is to be provided to monitor the sprinkler system and to activate notification devices within the ADU (780 CMR 34 Section 904.2), as the residential floor becomes three stories above the lowest level of exit discharge (780 CMR Section 907.2.9.1; and 780 CMR 34 Section 1011.6). Heat detectors with sounder bases are to be installed within 6 feet of the entry door in the non-ADU dwelling units, and low-voltage fire alarm smoke detectors are to be installed at each level of the common stairs connecting the units, including within the basement. The overall intent remains to provide robust common stair detection while using entry-adjacent heat detection and/or sprinkler activation for warning/activation without relying on in-unit smoke/CO devices to drive building-wide response.
Scenario 5: Existing three-family (three-story), convert unfinished basement into an ADU
For an existing three-family, three-story building where an ADU is created by finishing an unfinished basement, the property becomes a four-unit building. In this scenario, it is required to provide a sprinkler system per NFPA 13R within the ADU and also within the connected/affected common stairs serving the ADU. As with other multi-family conversions, the sprinkler scope extends beyond the unit itself because the common stairs are essential to safe egress for multiple dwelling units.
On the fire alarm side, On the fire alarm side, a manual fire alarm system is to be provided to monitor the sprinkler system and to activate notification devices within the ADU (780 CMR 34 Section 904.2), as the residential floor becomes three stories above the lowest level of exit discharge (780 CMR Section 907.2.9.1; and 780 CMR 34 Section 1011.6). Heat detectors with sounder bases are to be installed within 6 feet of the entry door inside the non-ADU dwelling units (including per 780 CMR 34 Section 307.2.1, where applicable), and low-voltage fire alarm smoke detectors are to be installed at each level of the common stairs connecting the units. The intent is that the common stairs receive reliable detection coverage, while entry-adjacent heat detection and/or sprinkler activation provides an additional layer of warning/activation without relying on in-unit smoke/CO devices to drive building-wide response.
Code interpretation note: 520-Hz low-frequency audible signal requirement in sleeping rooms
A code interpretation issue that frequently arises in these R-2 ADU projects is the applicability of the 520-Hz low-frequency audible alarm requirement in sleeping rooms. Under the Massachusetts Building Code, where a fire alarm system is required in Group R-1 or R-2 occupancies, the audible alarm signal in sleeping rooms is required to be a 520-Hz low-frequency signal complying with NFPA 72 (Section [F] 907.5.2.1.3.1). Further, in sleeping rooms of Group R-1 and R-2 occupancies that are required by Section 907.2.8 or 907.2.9 to have a fire alarm system, the audible alarm signal activated by single- or multiple-station smoke alarms must also be a 520-Hz signal complying with NFPA 72 (Section [F] 907.5.2.1.3.2). Where a sleeping-room smoke alarm is unable to produce a 520-Hz signal, the code permits the low-frequency signal to instead be provided by a listed notification appliance or a smoke detector with an integral 520-Hz sounder.
At present, there are no approved 120-volt single- or multiple-station smoke/CO alarms on the market that can produce a compliant 520-Hz low-frequency signal while also meeting Massachusetts acceptance criteria for these applications. As a result, standard residential 120V smoke/CO alarms—while acceptable in many one- and two-family residential applications—may not be acceptable for proposed sleeping rooms in projects where the building code requires 520-Hz notification. To comply with the code intent, a fire alarm–based detection approach is used within ADU sleeping areas. This typically includes smoke detectors within bedrooms and smoke/CO detectors outside of bedrooms, with each detector provided with a listed low-frequency (520-Hz) sounder base to achieve compliant low-frequency audible notification.
Engineering Fire Protection LLC has been serving homeowners, developers, and architects throughout Massachusetts—especially in the Boston area—for over 10 years. We specialize in ADU-related building code consulting, fire protection engineering, and fire alarm system design. If you are planning an ADU project and need assistance, please feel free to contact us by contact@engineeringfireprotection.com. We would be happy to help.