Single Stair Buildings for San Francisco: The Key to Building Small Scale Infill Housing

San Francisco is in the process of rezoning the city to allow for more housing in neighborhoods that have seen very little development in the past few decades. Because these areas are mostly built-out with existing structures, assembling large development sites is not likely to happen. Smaller scale development needs to work within the existing pattern of small lots. The building code needs to be updated to allow for this type of development.

Why are small residential buildings hard to build under the International Building Code?

While we call it the International Building Code, it’s an American invention and it isn’t used outside of the 50 states, the District of Columbia, Guam, Northern Marianas Islands, the U.S. Virgin Islands and Puerto Rico. This code is the model for the state level codes that are used to permit new buildings. Unlike codes in almost all other countries, it automatically requires two stairs if you are building over 3 stories tall (and limits three story buildings to four units per floor).

Adding two stairs to a four to six story building on a 25′ wide lot in San Francisco takes up an enormous amount of space, especially when combined with a corridor and an elevator. This reduces the number of units you can build by forcing the building to accommodate a large amount of space that can’t be lived in. It also detracts from the quality of the units that are built but reducing the width of units, and in many cases taking up exterior wall area that would be better off used for living space with windows.

Are single stair buildings common elsewhere in the world? What are the benefits?

Yes. Most other countries allow single stair buildings. They are often called “point access blocks” (which is a term favored by single stair advocate Michael Eliason of Larch Lab). You can see examples in Germany, Austria, France, Mexico and Japan (and many more). Even within our own country, Seattle modified the building code to allow single stair buildings up to six stories. What are the benefits?

1. More access to light

• By allowing for a central stair, bedrooms and living space can be pushed to the exterior and full floor units in a small building can get more exterior wall.
• In larger single stair buildings, units can be arranged around a small central core instead of along a double loaded corridor. Double loaded corridor buildings often result in very deep units that put very little living space near the windows.

2. Safety Considerations

• Single stair buildings are fully sprinklered and allow for shorter travel distances to get out of the building, allowing people to escape more quickly. Putting height limits on single stair buildings mean that fire department ladders can still reach the upper floors.
• Large double-loaded corridor buildings allow for 50′ long dead end corridors and have many more units sharing the same stairs.
• Seattle has allowed single stair buildings for years without safety issues, and Seattle is very similar to San Francisco: it is full of dense older neighborhoods, has hilly topography and is also in a seismic zone.

3. Design and Aesthetic Improvements

• It is easier to design larger family-friendly units with less space devoted to vertical circulation.
• More windows if exterior wall area is not taken up with fire stairs.

4. Construction Cost and Density

• The cost of building every square foot of residential space is lower if you are spending less money building stairs and corridors. The efficiency of the building (residential area/leasable area vs circulation and everything else) is higher.
• More units per lot are possible, making a more efficient use of sparse urban land.

Why is legalizing single stair buildings important in San Francisco?

San Francisco is divided into very small parcels by North American standards. The typical residential lot is 25′ x 100′, and even on major corridors it is common to see this dimension. Even a double lot that is 50′ x 100′ is very small. Fitting two stairs into a building where your typical floor might only be 1,800 square feet is nearly impossible. The San Francisco Planning Code is being modified to allow for taller buildings, but unless the Building Code is modified to allow for taller single stair buildings, it is very unlikely the zoning capacity will ever be used.

How do single stair buildings allow for more affordable housing?

Single-stair buildings allow sites to be developed that would otherwise be infeasible due to cost concerns as they would let you put 8 or more units on a lot that previously may have only held 3, or may have been used as a small parking lot or one story commercial space. In the case where the building is replacing a single family home, the “floor” for the land basis is the value of the home (which is typically at least $1.2 million in San Francisco). If the code realistically only allows for four units, the site is unlikely to ever be redeveloped. The potential home builders are competing with single family home buyers for the site and you’d end up paying at least $300k per door for land to build a new building.

This also means that for Affordable developers interested in doing small scale projects (like Community Land Trusts or non profits like Habitat for Humanity), they can get more economy of scale on small sites. In San Francisco, the SFCLT has bought existing small apartment buildings but this could create an opportunity for them or other similar organizations to build from the ground up.

How would single stair buildings become legal in San Francisco?

Either the San Francisco Building Code would need to be modified (this is the amended version of the California Building Code that is based on the International Building Code) or a bulletin signed by the Department of Building Inspection and San Francisco Fire Department would have to be issued detailing the code equivalencies required for this type of building.

Conclusion

Single stair buildings are a valuable housing type that is common in cities across the world and they are the best way to add density on small lots. San Francisco can learn a lot from both the international community and our friends in Seattle and create a framework for allowing these buildings here.

More info on single stair buildings:

• Greater Greater Washington: “How Single Stair Buildings Can Improve Fire Safety”
• The Urbanist: “Seattle’s Lead on Single Stair Buildings”
• Architectural Record: “Exit Strategy: The Case for Single-Stair Egress”