In the market for a mansion off Fifth Avenue? Is your last name Carnegie or Vanderbilt? Well, unfortunately, you’re about a century too late. But thanks to one innovative developer, you may have a chance to own one after all.

Three years ago, Mitchel Maidman’s Townhouse Management bought two large townhouses at 35 and 37 E. 63rd St. And then he did something unusual: He combined them.

What sits on the space today is a nine-story, 41-foot-wide, 20,000-squarefoot uber-mansion in which to hang your art collection, install a ballroom, or just entertain Nick and Daisy.

Mr. Maidman describes the circumstances of acquiring the properties as “fortuitous,” despite the difficulty of negotiating deals with all the tenants and acquiring the litany of permits for an endeavor like this, in a land marked district.

The property boasts 3,000 square feet of exterior space, 12 terraces, 16 wood-burning fireplaces, and two apartment-size elevator shafts.

The asking price is $29.5 million. But, Mr Maidman says, “When you have people paying $75 million for a house in Palm Beach, something like this on East 63rd Street is a steal.” The Corcoran Group lists a property at 9 E. 62nd St. for $23 million, but that property has only 11,600 square feet. The building at 5 E. 64th St. is listed at $32 million with 12,156 square feet. Woody Allen sold his Georgian 22-room townhouse on 92nd Street last year for $25 million. The 63rd Street property is unique, however, for its vast 41-foot width.

Sybille Novack of Sotheby’s Realty, one of the brokers handling the property, expects buyers to hold a net worth of about $150 million, although prospective buyers are not obliged to disclose their worth. “It’s a very international clientele, for people who have needs besides private shelter – a huge art collection, people like that.”

Mr. Maidman says the buyer would be “a very unique customer. I would say the economy has very little effect on the customers we’re looking for … somebody who is very wealthy, who does a lot of entertaining, who needs lots of room to hang their artwork.”

So, while the price of 35 E. 63rd St. is a relative bargain for those types of buyers, it comes with an extra fee: patience. The property remains an unfinished, or “white-wall,” property. Whoever decides to purchase it will have to put in a good amount of custom design work. The property, on the market since June, has been attracting attention, but given the difficulty of its concept, it’s not likely going to have many imitators, Mr. Maidman cautions: “I don’t think it’s a growing trend. People are buying very large houses throughout the country. Large apartments are a growing trend in the city. This is a very unique property with a very unique approach.”

And while the aforementioned properties are in the vicinity of the building, there are no properties on the market of this size, especially in this area.

“Not in this gold coast location,” says Ms. Novack. “Sure, there are commercial, but in residential, none that I know of. The only other possibility would be maybe to buy several floors downtown in a new loft building,” she says.

The majority of large-scale townhouses, Ms. Novack says, are around 25 feet in width, and top out at around 9,000 square feet. Most intriguing about the space is that a space 20 feet deep was dug below ground, creating a lower atrium level of about 4,000 square feet and ceiling height of about 20 feet, establishing an area, which, according to Ms. Novack, would likely be used for a spa or ballroom.