For five years, Erin Feher Montoya and her husband, Danny, relished the one-bedroom apartment they shared in San Francisco’s Tenderloin District. Then they had a daughter. Instead of fleeing for the suburbs, they tweaked their compact setup to make room for three. To double the square footage of their bedroom, Danny, a professional woodworker, built a sleeping loft accessed by a ladder. The crib for 1 1⁄2-year-old Orion, also designed by Danny, has storage bins below.