Taylor Morrison, a housing developer based in Arizona, was set to break ground on a 304-unit condominium development in Sunnyvale, near San Jose, when the bottom fell out of the housing market in 2007. The company went back to the drawing board, and last month it gained approval for a drastically different plan: a town house project aimed at extended families, where children, parents and grandparents can all live comfortably under one roof. Such multigenerational housing is specifically aimed at the booming immigrant population in the Bay Area, and is emerging as one of the few growth niches in a moribund housing market.