Condo conversions, in which rental buildings are converted into condos, are all the rage in New York City these days lately, according to industry experts.

That's no surprise in a city where undeveloped land is as rare as a pro-bono broker.

Jodi Stasse, managing director for new developments at Citi Habitats, which is currently working on an Upper West Side rental-to-condo, said conversions are attractive to developers because of the low inventory in the city real estate market and the lack of available land to build on.

Plus, since the rentals are already standing, "you can probably deliver the product to the market quicker than you can from building from the ground up," she said.

Karen Mansour, Executive Vice President of Douglas Elliman Development Marketing, which is the exclusive sales and marketing agent for The Leonard, a condo conversion at 101 Leonard St., said souring land prices are driving the conversion popularity. Buyers are thrilled because they get new apartments in grand old buildings.

"You can take these historical buildings and convert them into modern masterpieces, so the mix of the old and the new is really appealing to New Yorkers," she said.

The Leonard was built in designed and constructed in 1904 by legendary architect Frederick C. Browne. It launched sales in July 2013, and is currently 98% sold. Its lobby alone reflects its old-world grandeur, with chandeliers and a 15-foot ceiling. However, it offers modern amenities, like a rooftop retreat perfect for grilling up some barbecue and a fully-equipped fitness center. Most of the apartments here are already sold.

Another historical conversion is Rutherford Place, a landmarked building at 305 Second Ave.

Though Rutherford Place previously sold 30 condos between 2006 and 2007, the balance remained rentals until October 2013 when Rutherford Place officially launched condo sales.

All of the apartments are different from each other, but have ceilings that stretch up 18 to 20 feet. They are coupled with top-of-the-line, modern kitchens, according to Richard Cantor of Cantor-Pecorella, the exclusive sales and marketing firm of Rutherford Place.

"The units are very desirable and when you combine that with prices that are in the $12, $13, $1400 per-foot range," for 1,000-square-foot one-bedrooms and $2,000 square-foot two-bedrooms, Cantor said, "we're able to offer the buyer real value and still do a good deal for the developer, so both sides benefit from the transaction."

Jarrett White, vice president of marketing at Macklowe Properties which represents two conversions, 737 Park Ave. and 150 E. 72nd St., agreed.

"Apartment hunters are able to benefit from having the design of a pre-war building while enjoying modern amenities and technologies," he explained.

For example, 150 E. 72nd, which was built in 1913, was converted in late 2013 and is now more than 60% sold. It has Tennessee marble flooring in its lobby, the same marble used to build Grand Central Terminal. It also has a fitness room and a children's playroom for modern families.

"The trend will continue at a steady pace, especially in landmarked areas like the Upper East Side, where developers are less likely to find open space to build on," he said.