San Francisco will loosen a building requirement that housing advocates say has made it harder to construct apartments on small or irregularly shaped lots, opening a path for some mid-rise projects to use one interior stairway and a second exit outdoors. Mayor Daniel Lurie and the Department of Building Inspection are set to issue the rules Friday, Oct. 9. Four-story buildings may use one main stairway and a secondary exit such as a fire escape. Five- and six-story buildings may pair an interior stairway with an exterior staircase. Buildings taller than six stories will still need two stairways. Builders must submit plans for approval by the department, and they may appeal if a proposal is rejected. The current requirement generally calls for two interior stairways in buildings of three or more stories. Supporters of changing it say that arrangement can force long hallways and awkward floor plans, consuming space that could otherwise be used for homes and limiting natural light and ventilation. The change is intended to make smaller multifamily developments—often described as “missing middle” housing—more feasible. Such projects can include four- or six-unit buildings on mid-block lots, rather than large towers or single-family houses. Research cited by advocates estimates that removing a second stairway can cut construction costs by 6% to 13%. San Francisco is facing rising rents, with increases of as much as 30% over the past year in some neighborhoods. Market-rate construction has also slowed sharply: just one such project is under construction in the city, while several others are expected to begin in the coming months. Lurie has pursued other measures to encourage development, including reducing fees and affordable-housing requirements, streamlining approvals and allowing greater height and density in parts of the city under his Family Zoning plan. Supporters say the stairway rules could help that effort by making smaller sites easier to build on. “This is the first one, and it’s exactly the type of infill project that San Francisco needs,” said Sean Keighran, president of the Residential Builders Association. He said a builder plans to seek approval for a four-unit project using the new rules, after not submitting a project in 12 years. Keighran called the policy a compromise that preserves fire safety while giving builders more flexibility. He said no single change will resolve the housing crisis, but several steps together could help bring smaller projects back to city neighborhoods. The policy makes San Francisco the second California city to adopt single-stairway reform, after Culver City. Los Angeles is drafting its own plan. Laura Foote, executive director of YIMBY Action, called San Francisco’s move a necessary step but said she would like the single-stairway option extended to buildings as tall as eight stories. She said many sites covered by Family Zoning are too small for two interior staircases, and argued that the policy should go further to have a greater effect. Lurie described the change as “an important step for unlocking new potential homes in our neighborhoods,” saying the city needs more housing as the cost of living rises.