You rarely see a newly built triple-decker in Somerville, and almost all the ones you do see are actually renovations of existing duplexes. Why is that? Part of the answer is that our building code makes it surprisingly expensive to build small multifamily housing.
In the US, there are two main building codes: IRC (International Residential Code) and IBC (International Building Code). IRC covers single-family homes, duplexes, and row homes, while IBC covers everything else. In other words, new triple-deckers and quadplexes have to follow most of the same rules as skyscrapers and office buildings.
A building that falls under the IBC is more expensive to build for several reasons, chiefly more complex rules for safety, accessibility, and permitting.
Sprinklers: A duplex doesn’t require a sprinkler system. Add a third unit, and all three now need commercial-grade sprinklers, at a cost of least $70,000, plus thousands more per year in maintenance. Sprinklers do improve safety, especially in large buildings where it takes more time to get outside in the event of an emergency. But for small and medium size apartment buildings, it’s more than sufficient to have fire-resistant building materials and hardwired alarm systems.
Stairwells: At four stories, the IBC code requires buildings to have two stairwells, which cost up to $400,000 to build and take up valuable interior space that could be used for more or larger homes. There is sizable research showing that for small buildings it is sufficiently safe to have a single stairwell, because occupants do not have to travel far to get to the exit. That research is also supported by codes from Europe and Canada — our “international” building code is almost exclusively used in the US — where decades of safety data show that taller single-stair buildings are perfectly fine. Fortunately, US code is slowly catching up to this research, and the 2027 model IBC will allow single-stair buildings up to four stories.
Elevators: Massachusetts has an admirable set of accessibility requirements for midsize residential buildings, but they become cost-prohibitive when combined with our design standards for elevators. Elsewhere in the world, it’s both legal and cost-effective to put a small elevator into an apartment building. In the US, we require extra large elevators with price tags to match, which winds up producing fewer apartments with worse accessibility.
Permitting: One- and two- family homes have a simple permit process: submit plans, get them checked, and adjust on site if needed. IBC homes require additional documentation and additional specialists, including elevator technicians, environmental engineers, civil engineers, architects, and so on. This high level of complexity also increases permit timelines. These factors make it hard for home builders to cover these upfront costs for smaller projects.
The combination of all these factors creates a system where the cost for a new triple-decker is significantly higher than the cost for a similar size single family home. This creates an economic system that disincentives small multifamily homes.
But there is hope for reform. A variety of other states and municipalities across the country have recognized this cost issue and have made changes to their building code and processes. By copying them, we can make it easier and cheaper to build multifamily homes. Somerville and Massachusetts can and must do better to solve our housing shortage.
This blog post is adapted from the fantastic work by Sam Naylor and Boston Indicators. You can read their full report here.