They’re not movin’ on up.

A brand-new, $28 million penthouse in the sky in Carnegie Hill was so badly built it will take $9 million worth of repairs to fix an array of serious safety problems and prevent the owners from moving in for at least a year, they contend in a lawsuit.

The descriptions of 1228 Madison Ave., a co-op between 88th and 89th streets whose facade was designed by famed architect Robert A.M. Stern, boast of “confident architecture” and “elegance.”

“Every detail, from the tone of the marble tabletops to the finish on the door hinges, has been carefully chosen to create a harmonious whole, which feels classic but magnanimous, an elegant setting to make your own,” gushed the web site marketing the Upper East Side building.

Construction began in 2019, and the family bought the massive unit spanning the 16th, 17th and 18th floors in October 2020.

But they now claim in a Manhattan Supreme Court lawsuit that the five-bed, 5.5-bath triplex — which includes eight private terraces and 360-degree views of Central Park — is an unsafe money pit.

The walls are not straight; the hardwood floors were not properly installed and need to be replaced; windows and terrace doors are not weathertight; and a two-story interior staircase was built with combustible wood and has a step missing, claim the owners, who are identified in papers only as 1228 Madison Trust.

Not to mention the kitchen has no heating and cooling ducts, a window is in danger of falling into a bathroom, and a slop sink meant to be in the kitchen was bizarrely installed near the grand entrance, according to the lawsuit against the developers.

The builders “made a series of conscious decisions to construct a purportedly ‘luxury’ building, and specifically the triplex apartment, in a substandard and subgrade manner,” the owners contend.

The owners are suing developers Abram and Scott Shnay, SLCE Architects and New Jersey-based engineering firm Icor Associates for malpractice, breach of contract and fraud, and demand a minimum $9 million in damages.

The triplex buyer “had his own professionals and had always intended to complete the apartment himself and should not now be heard to complain,” said Janine Getler, a lawyer for Shnay, who said the allegations are “without merit.”

SLCE and Icor did not respond to messages seeking comment.

A lawyer for the owner declined comment.