Sometimes all it takes is a little luck. For a young married couple, it came in the form of this rare find: a 19th-century, three-story, single-family home in the heart of Paris. The building was a charmer with good bones, but was in need of some serious care. In a vibrant retrofit by architect Pierre-Louis Gerlier that includes structural reinforcements, the reimagined design is set off with a new floor plan. The lower level now serves as a space for the couple’s children, with the public areas—including an open-plan living/dining room and kitchen—on the floor above. Upstairs, the attic has been transformed into a very large primary bedroom with a green-and-white bathroom suite. The living room (pictured) showcases the firm’s bespoke carpentry work with a beautiful, mossy-green built-in bookcase that frames a new fireplace, and a staircase surrounded by arched doorways that hold hidden storage. “We created visual breakthroughs in order to connect the different spaces,” says Gerlier. “The rounded arches are there to help magnify these moments.”