Tear down a party wall, and what’s left?

When developer Robert Kaliner found out the townhouse next to his renovation was for sale, he didn’t hesitate: He combined the two into a single 40-foot-wide megamansion.

RoundSquare Builders paid just over $18 million for the side-by-side properties—one once home to John Lennon, the other to composer John Cage—and after three and a half years of work, it will soon hit the market for a casual $75 million (listed by Matt Lesser at Leslie Garfield). If it sells for that price, it would set a record for a downtown Manhattan townhouse. Here’s how this epic renovation came together.

Q: Why did you decide to turn two townhouses into one? “Simple answer: These kinds of opportunities—a double-wide in a prime New York location—are unicorns. It was a gift, and we wanted to maximize that opportunity.”

Q: What were the main challenges? “You never know what the Landmarks Preservation Commission is going to approve. First, we wanted the houses to look identical at the top. Landmarks had us make them look a little bit different. In hindsight, they did us a favor. It’s a perfect mix of looking like one but a little bit not like one. It’s the perfect balance.”

Q: What challenges did you face in the interior? “Removing the party wall, the structural boundary between the properties. This required more steel for structural support than you could imagine. If you don’t use the best engineers you know, this is why you hear all the time of buildings cracking or coming down.”

Q: Did you preserve anything from the days when John Lennon and Yoko Ono lived there? “John Lennon’s spiral staircase was a no-brainer.”

Q: Why did you decide to keep the house under wraps during renovation? “It was not a stunt, like a restaurant opening that’s hard to get into. It’s because people don’t like looking at a house in the throes of construction. You just want to bring your toothbrush and move in.”

Q: Do you think it will sell for the full $75 million? “Comparable properties sold quickly with bidding wars, but the goal isn’t record-breaking. It’s ensuring the buyer loves the house.”