Dormer Regulations for Existing Buildings

Illustration of a single family house with a dormer, which is a structure that projects from a sloped roof to provide more usable space in a building's attic.
New dormer regulations allow for more space in one- and two-family homes

Dormers are structures that project from sloped roofs to provide more usable space in a building's attic. The image to the right shows an example of dormers.

In 2024, the City Council changed zoning rules to allow more one- and two-family homes to add dormers to the attic level of the building without needing Board of Zoning Appeal approval. These changes were proposed by the Board of Zoning Appeal and adopted by the City Council.

Please visit Ordinance No. 2024-9 in the Zoning Ordinance for the full regulations, as well as the City’s Design Guidelines for Roof Dormers. While the Guidelines are not requirements, they are a helpful visual tool to illustrate best practices for designing and constructing dormers in Cambridge.

Overview of New Zoning (2024)

Some older buildings in Cambridge are “non-conforming,” meaning they do not meet the current size limitations of the zoning district even if they were legal at the time they were built. Article 8.000 of the Zoning Ordinance allows for limited changes to non-conforming buildings so that building owners can adapt existing buildings to needs that change over time.

A dormer is an architectural term for a structure that is built on a sloped roof that changes the height and shape of the attic space within the building. This change creates more usable space for occupants.

Because the zoning rules for all residential buildings were simplified in 2025, many dormer additions could meet the current zoning district limitations. But if they don’t, a dormer could be added without needing Board of Zoning Appeal approval if it meets the following requirements:

  • It cannot be taller than the existing roof.
  • It cannot extend past the existing outer wall of the building.
  • There must be three feet and six inches (3’-6”) of space between the side walls of the dormer and the side edges of the roof.
  • It must have clear windows located on the front wall that take up 30% or more of the wall area. Windows cannot be located on the side walls of dormers.
  • The eave of the existing building’s roof must remain or be rebuilt so that it is continuous across the length of the wall below the dormer to provide visual separation between the dormer and the existing building.

Illustration of a dormer on a house with a sloped roof. Image says: Dormer side walls must be set back at least 3 feet and 6 inches from roof edges. Image says: Front wall of dormer must have 30% or more coverage of clear windows. Image says: Roof eve must be continuous across the building.

People who want to increase the size of their attic by constructing dormers can apply for a building permit from the Inspectional Services Department if they follow these rules. If a person wants to construct a dormer larger than allowed or if their building has further non-conforming features than allowed, they may apply for a special permit that would be reviewed by the Board of Zoning Appeal.