The Missing Middle of Somerville

Before we start, the city is still accepting responses to their Davis Square Neighborhood Plan Survey. If you would like to support more housing in that area of the city, we encourage filling it out before the deadline of this Sunday, September 27th, at 11:59 p.m.

Despite Somerville being the densest city in New England, our lots are not really that dense. They are mostly 1 to 3 homes on small lots, we just have a lot of them and few large parks or commercial districts. So in a weird twist, we lack a lot of “missing middle” housing that many other cities use to achieve density. This creates an opportunity for incremental growth in Somerville.

However, our small lots provide a strict constraint on missing middle construction: we cannot build the more common double staircase design of apartment buildings that are built today. But there are solutions. Using single stair construction and a few tweaks to the zoning, we can add more housing lot by lot without it feeling like a dramatic change.

A typical lot in Somerville is zoned Neighborhood Residential (NR), is between 2,500 and 4,000 square feet, and has about 2 homes on it. And, until recently, new construction was limited to just 2 homes. But for the past few years, it’s been legal to build a triple-decker plus a backyard cottage on NR lots for 4 homes total. But it has a lot of setback requirements and it’s often prohibitively expensive. As a result, we haven’t seen very many built, certainly not enough to make a dent in our housing shortage.

Adding just one or two homes to a lot is financially difficult because our shortage has driven up the cost of land. Combine that with high construction cost and often unprofitable work add just a few homes. To make denser housing practical in the real world, we need a different building pattern that can increment a little more. One that spreads the fixed cost of land across more homes.

That's where Single-Stair (or "Point Access") buildings come in. Most US building codes require two separate staircases in any multi-story apartment building, which eats into usable floor space and adds cost. Single-stair buildings, common across much of Europe, need only one, which frees up space for more, larger units. Under current Massachusetts building code, single-stair buildings can be built up to three stories. That's set to change soon to four stories. Boston Indicators has done substantial research in this space and produced several reference designs, like these, showing how different buildings would fit into lots like ours:

The plan on the left side, showing an 8 home apartment building, would suit a typical Somerville lot very well. And switching from 2 to 8 homes is a big enough difference that construction becomes economically feasible.

A larger change also makes Inclusionary Zoning (IZ) — the policy that requires a share of units in new developments to be set aside as affordable — more feasible. That’s an especially big difference for our NR zone, which currently has no IZ requirement at all: every new home in the majority of this city is market-rate only.

Building taller naturally raises concerns about shadows. But other aspects of zoning and building code can meaningfully limit that impact. A typical triple-decker is 3 stories tall with 9-foot floors, but it often starts a half-story above grade — putting its overall height around 30-35 feet.

Under current zoning, a new 4-story building must be at least 42 feet high: the front door must be at least 2 feet off the ground, each floor must be at least 10 feet high. Ground-floor entryways and 9-foot floors would make a 4-story building come in at around 36 feet — barely taller than the triple-deckers already on the block. That ground-floor entryway is also more accessible for people using strollers, wheelchairs, and crutches.

If we're willing to go past those shadow concerns, other changes — like allowing lot mergers, or letting single-stair buildings reach 6 stories — could push density even further. But even without those additional changes, there's a lot of room for Somerville to grow denser without fundamentally changing the character of its neighborhoods.

Our NR zone covers most of the city and hasn't meaningfully changed in 100 years. With a few targeted changes to our zoning rules, we can create the conditions for a lot more middle middle housing in that zone. As Somerville moves toward more upzoning under this administration, we hope city leaders consider how to bring more of the city into the effort to solve our housing shortage.