Paul Cremoux, an architect based in Mexico City, never thought he would build a house for his parents. “No,” he says, laughing, “but they insisted, because they loved the house I made for another client.”

Paul’s parents—Nina Wanderstok, a retired interior architect, and Raúl Cremoux, a retired writer and political consultant—were already feeling like the house where they had lived and raised their children was much too large for the two of them. When Nina and Raúl visited Casa CorManca, a recently finished project of Paul’s, they immediately fell in love with it. CorManca, a small residence of about 1,700 square feet, opened up the possibility that they could have their dream home without needing an expansive lot to make it happen. They decided to ask Paul to build a new house in the garden of their existing home.

“We share Paul’s ideas about the importance of contemporary, sustainable buildings,” Nina says.

Paul faced two main challenges: The first was a budget of just about $210,000. The second was the fact that he could build on only 60 percent of the available plot, due to a law in Mexico City that requires that a percentage of the land be left without buildings in order to allow the ground-water reserves, the source of much of the city’s drinking water, to refill. This meant Paul would have about 1,400 square feet for the footprint. “The limits can be very strict, according to the zone in which you’re trying to build,” Paul explains, “so we decided to take it a step further. That’s where the idea of building a cistern to take advantage of all the rainwater originated.”

The new structure’s deck, made of a wood-plastic composite, acts as a water collector. Underneath it, a receptacle made of concrete and finished with a VOC–free paint collects the water. When it rains, the water flows through a series of carbon-activated filters that enable the household to subsist on rainwater for all its fresh-water needs. Although this does have the added advantage of saving money (water bills are around $10 a month), Paul is more interested in its ecological benefits.

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The terrace quickly became one of the home’s main attractions, not only because of its sustainable qualities, but also because it offered a chance to expand the living space outside with a pair of sliding doors. “Because the house is so small, that was a fundamental design decision,” Raúl says. “Once you open the doors, the space doubles. It’s perfect for entertaining family and friends, which we love to do.”

Another interesting feature is the library, which occupies the space surrounding the staircase. “As my parents are of a certain age, it probably would have been better to make the house just one level,” Paul says. But because of the land-usage law, a two-level structure was the only way to create the room his parents needed as well as accommodate his dad’s sizable collection of books. “It was obvious the stairwell would be best suited for the books because of its double height,” Paul explains. “Everyone always asks how it’s possible we have only 170 meters,” Nina says of reactions to the 1,775-square-foot residence. “Really, it’s just the staircase, the open kitchen space, and the height of the ceilings that add up to make it feel huge.”

After eight months of construction, the Cremoux-Wanderstoks were able to move in. Now they rent out their previous residence, since, as Nina says, they “made sure that the new house did not take anything away from the old one. It has a separate entrance; it has its own garden. All of the old house’s essential qualities remain.”

“The house didn’t have a big budget, but it feels expensive, because of the high ceilings, the skylights, and the amount of light that comes in from outside,” Raúl says of his son’s design. “From the beginning, we knew it was going to be a very livable home.”