San Mateo County supervisors have approved the next phase of a project that promises to bring new affordable homes to San Carlos. The action allows MidPen Housing to begin designing a new housing community on three County-owned parcels in the 600 block of Walnut Street, a site near the city’s busy downtown and within easy reach of schools, jobs and transit options. Under the County’s development objectives, the project will include a minimum of 75 affordable homes.