The Upper East Siders are coming!

The border that once divided Carnegie Hill from Spanish Harlem has been blurred as residents of Park Avenue, Madison Avenue, and Fifth Avenue who once feared venturing above East 96th Street begin buying up multi-million dollar pads on the north side of the cross-town street.

One of the newest developments is 21 E. 96th St. at the northwest corner of Madison Avenue. The nine floor through luxury apartments boast oversized windows, a bathroom suite in every bedroom, wood-burning fireplaces, French oak hardwood floors, and black-and-white Italian marble. Prices for the units range from $4.2 million for a four-bedroom, 4.5-bath apartment to $5.7596 million for the same sized unit with better views, appliances, and finishes. Half of the building’s units are sold, according to Corcoran agent Sophie de Sanctis, who has the exclusive listing with fellow Corco ran broker Victoria Banks.

“Lower Carnegie Hill is getting so expensive that people are getting pushed north to find value,” said Douglas Elliman broker Karen Connolly, who recently sold a listing at 1158 Fifth Ave., at the corner of 97th St. The buyers of the “classic eight” pre-war apartment, which boasts a formal dining room with a wood-burning fireplace and northern exposures, are moving from the east 80s because they can get more value for their money.

“If this apartment had been below 96th Street, it would have cost another $1 million,” Ms. Connolly said.

The buyers, an investment banker and his wife, have one child in a private school on East End Avenue and another not yet of school age.

“It used to be that 86th Street was the cutoff for the neighborhood, then it moved to 96th, and now even that line is blurring,” Ms. Connolly said. “Despite people wanting to move uptown, it was an issue selling this apartment, with a lot of people asking whether the neighborhood was safe.”

The asking price for the unit was $2.595 million when it came on the market November 1, and demand was fierce, with 130 couples coming to see the listing before the buyers made an offer, Ms. Connolly said.

“Carnegie Hill is moving into that no-man’s-land that used to be Harlem, but now I’m not sure what to call it anymore. Maybe Carnegie Hill North, or CoNo?” the head of new development for Douglas Elliman, Andrew Gerringer, said. Mr. Gerringer helped develop the Gatsby at 65 E. 96th St. between Park and Madison avenues three years ago, turning it into a luxury condominium from a rental building with Gary Barnett of Intell Management. “We were the first to see this potential, and it has been a slow process moving Carnegie Hill northward because it is so difficult to find development sites in the area,” he said.

It has been easier to find space along the less posh area east of Park Avenue, and a number of buildings have been built along the north side of 96th Street between Lexington, Third, and Second avenues.

The most recent is the Related Companies’ development One Carnegie Hill on 96th Street and Third Avenue, next to the Islamic Cultural Center of New York. The units, which range in price from $390,000 for a 477-squarefoot studio to $2.11 million for a 1,704-square-foot three-bedroom apartment, are roughly 60% sold out.

“People feel the East 60s are too commercial, and Carnegie Hill is more residential because it’s where the schools are, and all the nice shops,” said Corcoran broker Carrie Chiang. “I have clients who refuse to look in the 60s and only want to live in the high 90s,” she added. Ms. Chiang is the exclusive listing broker for the Carhart Mansions at 3 E. 95th St., well within the Carnegie Hill border but helping to drive up prices in the neighborhood, leading to the area’s expansion northward, experts say. The prices of the condominium units in the building, which used to be the Lycee Francais French day school, range from $12.5 million to $21 million.