Staff analysis shows 6‑story as‑of‑right yields most units in residential districts; corridors drive larger totals

Community Development staff told the Cambridge Planning Board on Dec. 17 that a modeling exercise comparing six rezoning scenarios found the greatest immediate housing gain in the residential districts comes from a six‑story as‑of‑right approach, but that most capacity remains in the city’s squares and corridors.

Melissa, presenting the analysis requested by the City Council, said the team ran two parallel analyses: one focused on residential districts (the A, B and C residential zones) and another that tested higher heights in targeted squares and corridors. The residential analysis used a redevelopment‑likelihood rule that flagged a site as likely to redevelop when the allowed gross floor area (GFA) delta exceeded 4,000 square feet and the GFA‑delta/existing ratio exceeded 33 percent; corridors used a higher threshold (roughly 10,000 sqft and a doubling of capacity). Unit counts were converted at an assumed 1,000 square feet per unit.

Under those assumptions, Melissa reported the six‑story as‑of‑right scenario produced about 1,630 additional units in the residential districts by 2030—the largest residential outcome of the six scenarios. Alternative scenarios (4‑story baselines with select taller streets, inclusionary‑only height bonuses, or a 4‑story everywhere baseline) produced fewer units in the residential districts by 2030.

The squares and corridors modeling, where staff tested substantially higher heights in places such as Mass. Ave., Harvard Square, Inman, Alewife and Central, showed a much larger absolute contribution to housing: roughly 4,900 units by 2030 (about 830 affordable) and about 12,425 units by 2040 (about 1,940 affordable) under the higher‑height corridor assumptions. Melissa noted that, taken together, rezoning both residential areas and corridors could move the City closer to the Envision Cambridge goal of 12,500 units but that timing and funding mean the goal would likely not be achieved by 2030 under the modeled assumptions.

Melissa emphasized the analysis is planning‑level: "This is meant to be a planning level analysis. It's meant to compare zoning scenarios and options, so this should not be seen as a predictive tool or to understand what sites will develop or not." Board members used the presentation to test policy tradeoffs between production and urban‑design protections, and several asked staff to quantify canopy or private‑open‑space impacts under reduced setbacks.

The staff memo and slides shown to the board are part of the public record for the continued public hearing and will inform Council deliberations during pending ordinance‑committee sessions.