The Spanish architects José Selgas and Lucía Cano are known for using curved surfaces, bright colors, and high-tech materials to create attention-getting buildings. But they also know that the less you build, the less carbon will be emitted into the atmosphere, reducing buildings’ contribution to global warming. And that means that renovating, as opposed to tearing down and starting over, is the greenest thing an architect can do. Outside Madrid, the couple, whose firm is known as SelgasCano, practiced what they preach, turning a cluster of rudimentary farm buildings into a weekend compound for themselves and their two sons, doing as little as possible to the original structures. The pair met regularly with builder Luismi Quintana and carpenter Rubén Criba to discuss what materials they had on hand and how they could best employ them. Most of the wood came from the existing house. They raised some of the roofs (Selgas is six four) and added windows, sometimes chiseling irregular holes in concrete walls, then inserting panes of clear acrylic. But they stuck to the agricultural buildings’ footprints. “The scale of the house was really nice. We didn’t need anything else,” says Cano. Adds Selgas, “Wasting space is a problem for our society. You should create space you’re going to use.” When the four-year renovation was complete, a neighbor came to see it. Entering the building, Cano recalls, “she said, ‘But you have done nothing!’ I said, ‘That’s the greatest compliment.’” selgascano.net —F.A.B.