By Jeffrey M. Hugo, CBO, and Jeanna Garrido – NFSA

In 2025, several states and local jurisdictions have either adopted or are actively exploring policies that allow 3- and 4-unit residential buildings to be constructed under the International Residential Code (IRC), rather than the more complex International Building Code (IBC). This shift is often part of broader efforts to address the national housing shortage and promote “missing middle” housing—affordable, small-scale multifamily units that fit within existing neighborhoods.

Traditionally, the IRC has applied only to one- and two-family dwellings and townhouses. However, recent legislation in several states is redefining the scope of the IRC to include triplexes and fourplexes, in an effort to streamline design, reduce costs, and increase housing availability.

State-Level Activity

• Alaska – Under HB 80, the Alaska Housing Finance Corporation must adopt the 2018 IRC as the statewide minimum standard for residential buildings with 1 to 4 This takes effect January 1, 2026.
• Rhode Island – HB 5373 amends the state building code to apply the IRC to residential buildings with up to four units, promoting a consistent regulatory framework for small residential
• Texas – While HB 5148 primarily addresses stairway design in multifamily buildings and references the IBC, it reflects growing interest in aligning design requirements with IRC-style standards for smaller projects.
• Utah – HB 175 modifies the state construction code to allow 3- and 4-unit dwellings under the IRC. The measure generated debate over increased construction costs due to updated life safety
• North Carolina – The state adopted HB 488, expanding the IRC to include triplexes and quadplexes, effective January 1, This marks one of the first formal adoptions of 3- and 4- unit dwellings under the IRC.

Local Jurisdiction Activity

• Seattle, WA – The city’s Housing Task Force recommended allowing up to four units under the IRC, aligning with zoning reforms and reducing permitting burdens associated with the IBC.
• Austin, TX – Ongoing discussions around increasing residential density have included builder support for using the IRC for 3- and 4-unit projects, citing cost savings and process
• Dallas, TX – In April 2025, the city adopted a local amendment allowing up to eight dwelling units under an IRC-based “One- to Eight-Family Dwelling ” The policy permits up to four units per story, a single exit, and a total building size of 7,500 square feet. While intended to address housing supply, the move circumvents the IBC and raises significant concerns regarding life safety and consistency with national model codes.

• Memphis, TN – Memphis amended its local IRC to permit up to six dwelling units under the residential code (rather than the IBC), specifically targeting “plex” structures as part of its missing middle housing initiative. These “large homes” are subject to tiered fire protection requirements: buildings under 5,000 sq ft may use NFPA 13D sprinkler systems in exchange for 1-hour rated fire separations, while larger buildings between 5,000 and 8,000 sq ft require a full NFPA 13R system. These modifications were adopted as part of Memphis and Shelby County’s 2021 IRC code package and reflect an effort to increase density while maintaining a minimum level of life safety.

Broader Legislative Context

A recent query using Policy Note, NFSA’s legislative tracking tool, returned over many 2025 results related to legislative efforts seeking to expand the IRC to cover 3- and 4-unit dwellings. Of these, NFSA closely monitored 54 bills with potential direct impact on fire sprinkler requirements. These results underscore the growing momentum around this issue and its national implications.

Zoning Reform

While zoning changes are not a direct fire sprinkler concern, they are a critical first step toward expanding the scope of the IRC to include 3- and 4-unit dwellings. States like Oregon, California, and Washington have mandated zoning reforms allowing multi-unit housing on lots traditionally limited to single-family homes. Cities including Minneapolis, Portland, Sacramento, and Raleigh have followed suit locally. These reforms signal growing support for higher-density housing that may ultimately pressure building code changes to align with land use policy.

2027 International Residential Code (IRC) Development

As part of the 2027 IRC development cycle, Proposal RB1-25 sought to formally expand the IRC scope to include three- and four-family dwellings. Proponents cited urgent national housing needs, labor shortages, and the desire to offer more affordable housing types. They argued that allowing these dwellings to be built under the IRC, with less stringent sprinkler requirements such as NFPA 13D (instead of NFPA 13R or 13), would lower costs and incentivize development.

However, RB1-25 was unanimously disapproved (10–0) by the IRC code development committee. Opponents emphasized that only the ICC Board of Directors has the authority to change the scope of the IRC. They raised safety concerns, including the absence of enhanced life safety features to the IRC, even simple increases like additional handrails or increased egress capacity, despite the doubling of allowed units. Critics also challenged the affordability argument, noting that 13D sprinkler systems already cost only 1–2% of total construction value and that downgrading from 13R saves little but sacrifices protection.

While RB1-25 was denied in the initial hearing, the issue remains under active discussion in the current IRC cycle, with a public comment period and further hearings scheduled. Meanwhile, North Carolina has already adopted IRC provisions for 3- and 4-unit dwellings through legislation. Oregon allows accessory dwelling units (ADUs) under its IRC-based Residential Specialty Code but has not yet fully adopted triplexes or fourplexes into the IRC framework. In Ohio, no formal adoption has occurred, though the topic has been discussed as part of broader housing reform conversations. These developments may increase pressure on ICC to take up the issue at the policy level.

Fire Sprinkler Concerns

A key concern with these IRC expansions is the reduction or elimination of fire sprinkler requirements, often justified as cost-saving measures. A notable example is Iowa SF 310, which proposed prohibiting mandatory sprinkler installation in attached single-family residences with fewer than seven units. While such proposals are often positioned as housing affordability solutions, they pose significant risks to life safety, especially as the number of dwelling units increases without corresponding protection.

Conclusion

Efforts to expand the IRC to cover 3- and 4-unit dwellings are gaining traction nationwide, driven by housing supply challenges and affordability goals. However, many of these initiatives come with trade- offs in fire safety, particularly related to sprinkler system requirements. NFSA continues to monitor this issue closely and advocate for maintaining consistent, effective fire protection standards—regardless of the number of dwelling units or the code under which they are built.