A big chunk of why is the % of floor area that is used for housing versus “support”. Basically a single stair layout gives you 90-95% floor efficiency, and a do

A big chunk of why is the % of floor area that is used for housing versus “support”. Basically a single stair layout gives you 90-95% floor efficiency, and a double stair is closer to 80-85%. People don’t rent common spaces like stairs, so you can bring the value of a building up by ~10% by allowing each apartment to have only a single exit staircase. Also if a lot is small (like 20ft wide or less), you have to compromise the design. For instance, instead of having full-floor units that get cross ventilation (windows at front and back), you may need to have duplex units that take up 40% of the front or back (so only get one side of the building).