In 2010, Phu Hoang and Rachely Rotem bought a two-family Carroll Gardens building constructed in the 1930s. They live in the duplex on the bottom two floors and rent out the third. But the couple, who are both architects — they founded the firm MODU together in 2012 and won the Rome Prize in 2017 — always wanted to expand the house. The question was how they would do that.

“This project had been in our studio for a while,” Hoang admits. It took them years to come up with a plan for creating more space owing to zoning logistics, until the solution was realized in what they are calling “Mini Tower One,” a slightly taller addition to the original building that extends over the rear like a sleek backpack.

“This whole idea started during COVID,” Rotem says, as we stand in the living room where the back wall of the house once was. Their idea was an extension that would enlarge each floor of the building while enabling an indoor-outdoor experience. “This is a birch tree,” Hoang points out in the back, “and you can see the relationship to some of the windows — the branches are, like, five or ten feet away — and the idea is that each level gets a different experience with the tree.”

Large glass doors added as part of the extension just off their bedroom on the second floor can be folded back to allow the room to open completely (the floors are weatherproofed, as is the furniture).

Downstairs, off the living room, is another room that opens up; the curtains can be drawn when it’s used as a guest room. Mirrors are used strategically within the space. “We are trying to mark what is new and what is old and to balance the light within,” Hoang says.

It gives one the feeling of being in a very elegant tree house. And the birds love it.