Cambridge committees weigh zoning stepbacks to protect rooftop solar, agree to continue study

Cambridge City staff and councilors spent more than an hour on Sept. 16 examining whether zoning changes could protect existing rooftop solar from shading by new multifamily buildings, ultimately agreeing to continue study and outreach rather than immediately pursue a citywide amendment.

The joint Housing and Health & Environment Committee heard a presentation from Jeff Roberts, director of Zoning and Development, who said the city’s modeling showed the largest solar impacts occur when very tall buildings (modeled at 74 feet) are sited close to lower buildings. Roberts told the committee that a targeted dimensional approach — for example adding an extra 5-foot north-side stepback on a building’s fifth story and another on a sixth story — could reduce top-floor area in extreme examples by 9 to 19 percent and total building floor area by about 3 to 6 percent.

“Those setbacks would provide protection in cases where there would be a significant impact,” Roberts said, while warning that such rules would be uniform (apply across the city) and could increase structural complexity, engineering costs and the risk that some projects would instead be built at four stories rather than five or six, reducing inclusionary affordable units.

Julie Wormser, the city’s chief climate officer, framed the tradeoff between individual solar installations and citywide decarbonization strategy. She said small residential rooftop systems are important to building owners and energy justice, but they contribute a modest share toward the city’s overall emissions reductions. “Small residential rooftop solar makes a big difference to the building owner,” Wormser said. “But in terms of our citywide goals of getting the whole city off fossil fuels, small residential solar doesn’t make a big enough difference.”

Residents who testified during public comment pushed staff and councilors to protect the community’s existing solar investments. Liz Byron Loya, who said she lives on Field Street, pointed to the city’s July 2025 urban design guidance recommending stepbacks and told councilors, “If developers say this can’t be done, it’s not true.” Other commenters said seasonal shadowing and rooftop mechanicals were not adequately reflected in the staff presentation and urged either setback protections or other mechanisms to grandfather or compensate existing systems.

Councilors pressed staff on methodology and next steps. Staff said the consultant’s analysis used total annual solar radiation (an aggregate measure) rather than single-date shadow studies and acknowledged the packet’s 5,000-square-foot-lot example represented a worst-case percentage impact; larger lots would show smaller percentage reductions. Staff reported roughly 800 to 1,000 residential photovoltaic systems citywide, with about 800 on single- and two- or three-family buildings, but cautioned permit data can include multiple filings per address.

Rather than adopting a zoning amendment immediately, staff recommended and councilors supported continuing the review, expanding homeowner outreach, modeling at least one larger-lot example, and reporting back as part of the city’s annual housing review (anticipated in the first half of next year). Councilor Azim and others asked staff to explore incentives and “carrot” approaches alongside regulatory options to avoid undermining the financial feasibility of inclusionary developments.

By the end of the joint meeting, councilors voted to adjourn; the recorded roll call showed the motion carried in both bodies. No zoning amendment or final policy decision was adopted at the meeting; committee chairs said more evidence and public engagement will guide any future proposal.

What’s next: Staff will refine modeling (including larger-lot scenarios), reach out to homeowners and the city’s climate committees for input, and return to the council with further analysis and options before any formal zoning filing.