Even though it’s on a desirable street, this brick townhome built in 1899 on New York City’s Upper East Side doesn’t appear unique.

But once you get past the stately facade, you will be simply amazed by an expansion that’s invisible from the street.

In 2006, a modern five-story extension was added to the back of the building by architect William Georgis. In a city where space is a coveted commodity, the home’s depth grew from 48 feet to 60 feet at its base. The architect’s amazing glass-and-steel addition was featured in the book “Make It Fabulous.”

And the history of this home is equally worthy of a book. Famous writers, painters, and actors, including Edward Albee, Toni Morrison, Gloria Swanson, Cy Twombly, and Nell Carter, have walked through the front door. Swanson reportedly used the elevator hidden behind a wall tapestry for privacy from her fans.

Dr. Ward F. Cunningham-Rundles, who’s owned the home since 1974, treated patients in the building for nearly 50 years.

“When he first purchased the home, it was nothing like it is now,” says Michelle Bourgeois of Sotheby’s International Realty. She and agent Gina Kuhlenkamp listed the 5,400-square-foot townhome for $7,900,000.

It has seven bedrooms, five baths, and five half-baths. Each floor features a sunroom overlooking the garden. The top floor is entirely open and has a rooftop deck.

Other highlights include five working fireplaces as well as an elevator. The home’s distinctive mansard roof is an exterior standout that’s been well-preserved over the years.

The home is wired with Savant music, phone, intercom, and security systems.

“This home is set up seamlessly for entertaining,” says Bourgeois.

Below street level, there’s a full-height office floor that was excavated by Cunningham-Rundles for his medical practice.

Featuring skylights and crisp, white walls, the space could be turned into a day spa.

“It doesn’t have that institutional, commercial look to it for a live-work space,” Bourgeois says.

It could also be transformed into a media room, play room, or staff quarters.

“The house is ready for a new owner. It’s too much space for him,” Bourgeois says of Cunningham-Rundles. “He’ll be relocating his practice elsewhere in the city.”

Whoever buys it will be plugged in to a friendly group of neighbors.

The houses on this block “don’t change hands that often,” says Bourgeois. “All the neighbors know each other. It has a very neighborhood feel, rare for Manhattan.”