San Diego has emerged as a leader in the nationwide effort to reform local zoning laws to build more housing. But most of the new apartments going up are studios and one-bedroom apartments. Families with children often have to look to the suburbs to find two- and three-bedroom homes they can afford.

As San Diego seeks to encourage more family-sized apartments in its urban core, architects say the key lies not in the city’s zoning laws but its building code. Specifically, a requirement that any building above three stories have two staircases.

A growing “single-stair reform” movement across North America argues this rule — adopted in the early 20th century to allow for faster evacuation during fires — has outlived its purpose. Most countries in Europe, Latin America and Asia, and a few cities in the United States, allow single-stairway buildings of six stories or taller.

Proponents say fire prevention and suppression technology has evolved to the point that a second staircase doesn’t provide much safety benefit. California now requires sprinkler systems in all new residential buildings, meaning fires are extinguished more quickly. Regulations on fire-resistant furniture and building materials have also reduced the risk of fires spreading out of control.

Late last year, the San Diego City Council voted to offer relief from fees on certain three-bedroom apartments to encourage developers to include them in their projects. David Pearson, a San Diego-based architect, said single-stair reform could go much further toward that goal.

Pearson designed a three-story, single-stair apartment building that’s due to break ground this summer in the backyard of a single-family home in Grant Hill. Four of the new homes will have two bedrooms, plus a den space that could serve as an office or nursery.

Pearson said he and his client chose a single-stair layout because it allowed for a shared courtyard, the preservation of the existing house and more space devoted to housing rather than halls and stairways. Single-stair buildings can also allow more units to have windows on multiple sides, which can reduce electricity use by providing more natural light and ventilation.

“If we were to build a second stair and try to create more units, it would have taken over any leftover space,” Pearson said. “It very likely would have incentivized the owner to demolish the existing home and just do the biggest thing possible.”

Most dual-staircase buildings use a layout called a “double-loaded corridor.” Pearson likened the design to a hotel: two staircases on opposite ends of the building connected by a long hallway. Most of the units in these buildings have only one side that opens to the outdoors.

Last year, California passed a law that directed the state fire marshal to produce a study on the safety of single-stair apartment buildings above three stories. The study is due by the end of the year.

Tony Tosca, San Diego’s local fire marshal, said his initial reaction to the law was concern. Building fires can be chaotic, he said, and having multiple paths of ingress and egress can be valuable.

“People are going up there to do rescue and fight fires and set up their operations,” Tosca said.”People are also coming out, so there’s this competing factor. That’s a huge concern for me.”

Still, Tosca said he’s open to allowing taller single-stair buildings if they’re coupled with other life and safety regulations, such as limiting the number of units allowed on each floor.

“Housing is an important issue here in California, especially in San Diego,” Tosca said. “As long as there’s something that maintains that life safety aspect, we’re all in support of it. But we just have to make sure that it’s done the right way.”

One organization pushing for statewide single-stair reform is the Los Angeles-based Livable Communities Initiative.

The group’s policy director, Ed Mendoza, said in addition to offering better light, ventilation and open space, single-stair reform can lead to greater density within smaller buildings that fit better into a neighborhood’s architectural character. Today, developers often have to purchase multiple lots to have enough space for a project that makes economic sense.

“We don’t have to wait for large sites to get combined, we don’t have to have half of our neighborhood block torn for one apartment complex,” Mendoza said. “The impact (of growth) won’t be that sudden. It’ll be a very gradual thing.”

Mendoza said he has worked with officials in cities like Santa Monica, Burbank and San Luis Obispo to study local building code amendments to allow taller single-stair buildings. He and Pearson also had a meeting with an aide to San Diego Mayor Todd Gloria last year.

Pearson said he hopes Gloria sees the value in single-stair reform and directs city staff to study the issue more closely.

“I would like to see the city look at building code reform as a means to create good, safe family units that ultimately provide more freedom of choice for residents of San Diego to stay put in San Diego, not move out to the suburbs,” Pearson said.