From six stories above the corner of Cedar Street and Sixth Avenue, San Diego looked unaccustomedly quaint—an old-growth seaside town of mission-style churches and midcentury office towers strung along tidy streets leading down to a sunny harbor. The interstate, bane of the southern California metropolis, was just a block north, but on a Monday afternoon in early spring traffic was thin, so the freeway noise was hard-ly detectable.

Since March, this 3,500-square-foot rooftop prospect has been a private reserve for tenants of the newly built Cedar Gateway Apartments. The residents, installed in sixty-five units, have full use of the…