Sliding glass doors and a sunroom on the back of the house adds to the feeling that “when you are inside the house, you are really outside,” Mr. Judson said. “You can see into the entire front yard and backyard when you are in the house.”

Ms. Hammond, the landscape architect, said the indoor-outdoor trend came from California, and wasdriven by “people wanting more space to expand their entertaining.”

“We want a reason to be outside other than the sake of just being outside and gardening,” Ms. Hammond said. She uses hedges “to create the boundaries of an outdoor room” and provide enclosures, with trees for shade and “an overhead sense of protection.”

A curved bench under a colonnade in one Southampton flower garden she designed becomes a “separate garden room, like a library in the house, a place you go to do something different,” Ms. Hammond said. Vegetable gardens in raised beds, which her clients clamor for, are “like a pantry,” providing food for outdoor kitchens and outdoor dining.

For those with a vista, roof decks are in vogue. Mr. Kean installed a putting green and Adirondack chairs on his mansion’s roof. “Once you are up here, you want to have a cocktail,” he said.

At Ms. Breitenbach’s Sagaponack listing, the roof deck has a Jacuzzi, a kitchen and a fireplace. And in East Hampton, Peter Wilson, a retired corporate lawyer, and Scott Sanders, an interior designer, erected a giant lifeguard chair on the roof of their $16.9 million, 3.2-acre estate to see the ocean.

In the spring of 2016, Pamela Glazer, a Southampton-based architect, cut out half of a tall attic to create a roof deck as part of the $525,000 renovation of two 1960s prefab homes cobbled together in Hither Hills. She used board and batten on an exterior wall to create a cozy backdrop for a sectional sofa, and a bungalow-style railing so the owners would have a place to enjoy cocktails while gazing over their neighbor’s houses at the Atlantic.