The married architects José Selgas and Lucía Cano have become known for their luminous buildings, mostly made from translucent materials in bright colors and bulbous shapes. These include Second Home coworking spaces in Los Angeles, London, and Lisbon, Portugal; a school in Nairobi; and rec centers and meeting halls throughout their native Spain. But the couple’s weekend house—decidedly rustic, with barely a smooth surface in sight—couldn’t be more different from those widely published projects.