Don Dimster and his filmmaker brother, Dennis, had lived together on and off for nearly a decade when, in 2004, they purchased a 40-by-120-foot lot in Venice, California, just a few blocks from the beach. Eight years and myriad hypothetical schemes later, the Dimsters moved into the duplex that Don, an architect, designed. By this time, Dennis was married to Noreen Perez, an assistant film director, and Don to Lisa Turner Dimster, the design director for the outdoor clothing maker Aether Apparel; what might have become a pair of high-design bachelor pads instead turned into two family homes with considerable flexibility.

The building’s most striking feature, both inside and from the street, is a pair of glass-walled, suspended steel stairways that lead from each home’s living space to a shared 1,000-square-foot rooftop patio. The terrace provides ample communal space for the two couples, their dogs, and Dennis and Noreen’s new baby. And when each family opts for a bit of private time, the six-inch-thick cinder-block walls that define each of the three-bedroom, two-and-a-half-bath homes give them all the privacy they need. Don shares the story behind the Dimster duplex.

Don Dimster: The lot was empty when we bought it, but we found an undocumented basement with old Prohibition-era bottles and two steep staircases leading into a tiny room about four-and-a-half feet underground. Apparently, there were a lot of moonshine basements in Venice.

We considered every permutation for the building: three units, two units, a garden courtyard—but, in the end, we knew we wanted it to be a duplex, with my brother and me as the occupants. I wanted a building that could read as one but that had two distinct identities.

Because we wanted a garage, there wasn’t much space left on the ground floor, so it was important to have a nice entry that brings you up to the living area. The stair is a special element. We studied different configurations—next to the facade, perpendicular to the facade—but it became obvious the stair should become the facade. It animates the building’s exterior and shows life inside the house. Because of its shape, the stair is quite a nice sculptural piece.

The idea of a collective main space was always important. It’s actually an old idea; Le Corbusier was a proponent of it in many of his social housing projects, and the theory was that the collective space would always be much more grand and significant than individual spaces. As the building foot-print grew, the collective outdoor space moved naturally up to the top floor.

The deck is an access point for the two units. The other night, I went over to watch basketball and it was so convenient. Lisa and I can go hang out, watch a movie on their side, or meet up with them in the middle. It’s a nice balance between togetherness and privacy.

The second floor has 11-and-a-half-foot ceilings, is a simple L shape, and contains the kitchen, living, and dining areas. We didn’t want to waste duplicate space, so we built the dining room inside the kitchen.

My brother’s unit is a mirror of ours, so if you wanted to bust through and combine the whole shebang into one big building, the kitchen would be a logical place to connect. There are six inches between the two walls. You could turn one kitchen into a formal dining room and use it as the passage to the home’s other side. The project is really designed to be future-proof: One family could take over both units, two families could stay, or one could sell without affecting the other.