San Diego is mulling a change to its building code that supporters say would spur more family-sized apartments with better natural lighting and ventilation. The change has to do with the longstanding requirement in most of the United States that buildings above three stories contain at least two stairwells. The rule was adopted in the early 20th century as cities grappled with fire safety, though critics say the advent of sprinkler systems and other fire suppression technologies has made the rule obsolete.