Mayor Michael R. Bloomberg’s town house at 17 East 79th Street is the epitome of Upper East Side elegance: five stories of flawless Beaux-Arts limestone with 7,500 square feet of exquisite living space, all within steps of Central Park.

But for the mayor, it seems, the house has been a bit cramped.

Over the past two decades, in transactions that have gone all but unnoticed, Mr. Bloomberg has been buying up space in the building next door, knocking down walls and combining two entire floors along the way. He now owns four of the six apartments at 19 East 79th Street, a white 1880 neo-Grec co-op town house.

The additions have made his home far roomier, giving him an estimated 12,500 square feet of living space; it would probably fetch more than $30 million, even in the current down market.

“You’re talking about a really extraordinary space,” said Jacky Teplitzky, a prominent Upper East Side broker, when told of the expansion. “It’s really a mansion, not a town house anymore.”