There are several reasons for the flood of boutique buildings, including the fact that after the downturn, financing was tight and many developers couldn’t secure large enough loans to fund a big development. “It is also very difficult to find sites large enough to build big projects — they almost don’t exist,” said Miki Naftali, the chairman and chief executive of the Naftali Group, a development company. He added that boutique buildings are a favorite with developers because “they are less risky” — it is easier and faster to sell out a 25-unit building than one with 200 units.

While new condo developments often receive attention because of their high-profile buyers, lofty price tags and brand-name architects, the resale market for trophy mansions has also been surging.

“It is a very, very strong market,” said Paula Del Nunzio, a broker at Brown Harris Stevens who specializes in ultraluxury properties. She recently had a listing for a home in the low-$20 million range, and in the first two days it was listed, she had shown it to 15 potential buyers. Two offered the full asking price, and the eventual winning bidder paid $1 million over asking. “The key is to have something that is unique, and then you can put a very strong price on it,” Ms. Del Nunzio said.

But while sales of new developments and trophy properties are continuing apace, there is one sector that has “hit a wall,” as one broker phrased it. Since April, the market for high-end co-ops and older condominiums priced from $8 million to $15 million has slowed, some brokers said. “The week around Passover and Easter, suddenly things went quiet,” said Kirk Henckels, the director of private brokerage at Stribling & Associates. “The only explanation that I can come up with was that there had been a lot of pent-up energy, and it sated itself.”

Frederick W. Peters, the president of Warburg Realty, has seen a similar pause. “It is critically important to distinguish between the new-development marketplace and every other marketplace, because they are not behaving the same way,” he said.

Still, so much in the market is anecdotal, with one broker reporting a far different experience from that of her colleagues. Ms. Kleier, for example, recently listed a co-op on Park Avenue in the mid-$9 million range. “I put it on the market for 25 percent more than what the last apartment in the building sold for,” she said. “Ninety-three people came to the open house and we sold it at ask the first day.”

But even developers who are churning out new condominium buildings don’t believe prices will continue to rise at the same frenzied pace. “If you look at the escalation of sales prices in the last year to year and a half, I don’t believe we will see that same escalation over the next one to two years,” said Mr. Naftali, who has a new condominium project at 210 West 77th Street that is expected to come to market later this summer.

“Prices won’t go down, but a year from now they won’t be up another 25 percent,” he said. “When I underwrite a new deal these days, I don’t assume a major escalation in price.”