A few weeks ago, I watched a man who had spent much of his life living in doorways and cardboard boxes shamble into the sun-washed lobby of a new building on Hegeman Avenue in Brownsville, dig an I.D. card out of baggy jeans, swipe through the turnstile, and take the elevator up to a modest but clean room equipped with a bed, an ample window, a closet, and a kitchenette. He was home. Built by the nonprofit organization Common Ground and designed by Cook + Fox, the Hegeman demonstrates how much quality architecture a scant budget will buy. Constructed for a modest $43 million, it is the best building for blocks, a handsome brick arrangement of 161 energy-efficient studios, a gym, support-staff offices, and a computer lab, all wrapped around a landscaped garden. There’s no mystery to making low-cost housing this well designed: Hire better architects.