On a quiet cul de sac in suburban Los Angeles, the installation of Martin Guevara and Grace Kuo's prefab accessory dwelling unit, or ADU, caused quite a stir in the neighborhood. "It was like a block party," said Guevara, 68. "All of our neighbors came out to watch." Guevara was referring to a prefab ADU designed and built by Villa Homes; the 850-square-foot home was craned over the couple's existing home and placed into position in their backyard in the South Bay last year. "It was in two halves," said his daughter Melanie Guevara, 33, who lives in the ADU...