Massachusetts needs more housing, especially in walkable, transit-served, amenity-rich neighborhoods. Recent policy reform has focused on zoning, and these efforts are starting to generate new housing. But zoning is only one part of the rules that shape what housing gets built.

Building codes also play a key role, and they do so in ways that are often invisible to the general public. Two codes govern nearly all residential construction in Massachusetts: the International Residential Code (IRC), which applies to single-family homes, duplexes, and townhomes, and the International Building Code (IBC), which governs everything else, from triple-deckers to soccer stadiums. The shift from two units to three triggers an entirely different set of requirements, with important implications for housing design, feasibility, and cost.

To explore these issues, Boston Indicators partnered with Sam Naylor of the architecture and design firm Nominal, who served as lead author of this report, with editorial support from the Joint Center for Housing Studies at Harvard.