To maintain exclusivity and convenience via an attractive amenity, a select group of office buildings are choosing to install their own tenants-only fitness facilities.

These now include the Empire State Building, which has been creating its own vertical campus with many amenities; the far smaller, more boutique-sized 111-115 Broadway; the tony 510 Madison Ave.; and the new International Gem Tower.

At the International Gem Tower, which has a jewel tenants’ lobby at 50 W. 47th and a high floors’ lobby at 55 W. 46th, there is a private, tenants-only 2,000-square-foot gym. It includes locker rooms, showers, steam rooms and bike storage.

On a tour earlier this year, Raizy Haas, senior vice president, project management and development for Extell Development, said it will also have a “quiet” room for yoga.

Extell developed the 34-story, high-security mixed-use office condominium building in part for the diamond, gem and jewelry trade. The separate office “building” on the top floors, entered at 55 W. 46th St., is being sold to SL Green Realty Corp., which will take over all operations, including the gym.

David Falk, Tri-State president of Newmark Grubb Knight Frank, said that during a space search “when there’s a tie” between spaces, a gym can be the tie-breaker, “but it’s not going to be the driver of the decision.”

Richard Cohen, president  of Capital Properties, however, said amenities, in general, often drive the leasing of office space.

Sit fireside inside the gym at 115 Broadway, set to open by the end of the year.Evan Joseph Images

His 111-115 Broadway, a k a Trinity Centre, a historical property near Trinity Church, is installing, among other features, a full floor of white-boarded conference rooms, a golf simulator and a new gym on a top floor.

“We’re putting in a health club on the 22nd floor for tenants only,” said Cohen. “It’s not a small little health club in a basement  — it is well lit and sunny, 4,000 square feet and elaborate.”

The building’s leasing agent, Bradley P. Gerla, executive vice president at CBRE, says the gym is already helping attract tech and advertising tenants to the downtown building.

The gym will be open by the end of the year.

“Tenants don’t have to leave the building and can work out before work or during lunch,” said Gerla. “That’s the beauty of it.”

Swim a few laps at Boston Properties’ 510 Madison Ave., which sports a 7,000-square-foot fitness facility.

Boston Properties’ 510 Madison Ave. ups the exclusivity ante by offering only five passes per floor to its 7,000-square-foot fitness facility. The amenity is billed at $1.50 per foot, said its leasing agent, Paul Amrich, vice chairman of CBRE. Where a floor is shared by two tenants, three might go to one and two to the other, he said.The gym was designed by developers Harry and Billy Macklowe prior to the luxury office building being sold to Boston Properties. It includes a 50-foot lap pool, marble vanities and wood-paneled locker rooms fully stocked with top-of-the-line products.

Currently, 510 Fitness is managed by Plus One Health Management. Along with the pool, it has a weight room and cardio machines, and offers personal training, yoga and Pilates.

“They implemented the highest-grade gym in an office building,” said Amrich. “The lap pool is an extra benefit and it’s a nice architectural piece. It’s been a real draw and has assisted with the lease up. A lot of tenants in the building are very serious about their workouts so having it, and being private, is a real benefit.”

The 35-story tower now has roughly 150 passes to the gym handed out, but has the capacity to go up to 200.

“The reason we kept it to that number is so the gym is never crowded. You will only see five or 10 people working out,” said Amrich.

Broker Daniel Posy, senior managing director at Savills Studley, who represents many hedge fund clients, said, “It’s a great amenity for my clients who would otherwise put in their own gyms.”

Posey explained that getting permission from a building owner to install a gym in a tenant’s own space is a “nightmare” and “causes messy lease negotiations.”

Gyms on tenant floors can become noisy for other tenants, Posy explained, especially when employees drop weights. It also adds extra plumbing for showers.

“Tenants will not put in a health club because it’s too costly,” said building owner Cohen. “It’s really the job of the building to put in those amenities.”

There’s an expansive, in-office fitness center fit for the giant Empire State Building (left). One of the bathrooms inside the Empire State Building’s expansive, 15,000-square-foot, tenants-only gym, which features 17 Life Fitness treadmills, crosstrainers, stairclimbers and bikes (right).Empire State Realty Trust

The gigantic Empire State Building, which is creating a vertical urban campus, recently opened a health facility in its concourse that’s fit for an 800-pound gorilla.

The 15,000-square-foot, tenants-only gym, the largest tenants-only facility in the city, is operated by Plus One.

17 Life has installed its own treadmills, crosstrainers, stairclimbers and numerous bike configurations along with strength equipment.

Although even lowly employees can join for $35 per month, there is a separate private executive fitness section with three changing cabanas.

“This is very, very high-end and very lavish,” said Tom Durels, executive vice president and director of leasing and operations for ESB’s owner, Empire State Realty Trust.

“Our tenants will never wait for a treadmill or a shower. There is an abundance of showers in lavishly appointed locker rooms and a very high-quality experience for the exclusive use of our tenants,” Durels said.

He called the $35-per-month fee a “bargain,” and says the company will make its money through the lease up of the building, and attracting and retaining tenants at higher rents.

The health club opened earlier this month and hours are being rejiggered as use patterns are monitored.

Other facilities include a tenants-only conference center, and on-site food options, including the new State Grill & Bar with private executive dining and private event space.

“It’s all about creating a very remarkable, very unique experience housed in this architectural gem,” said Durels.

The success of the tenants-only gyms as a private amenity is giving other owners the incentive to add facilities to their own buildings. Amrich, of CBRE, will soon launch marketing for 222 E. 41st St. — which will be entirely vacant when its only tenant, law firm Jones Day, moves downtown — and will soon feature a high-end athletic club.

Built in 2002, the 400,000-square-foot tower has high ceilings and floor-to-ceiling windows. In consultation with Amrich, the owners, Atlanta-based Columbia Property Trust, decided to fill the second floor with amenities.

Amrich said in addition to the athletic club, which will be just for building tenants, it will have a high-end food service similar to a dining club for the building’s executives.

A luxurious lounge and library- like environment will also have “cool” meeting areas, he said.

The Midtown building will also have rents averaging in the $70s per foot.

While many are now available, the very first non-public office fitness center was installed by Hartz Group at 667 Madison Ave., which is known as one of the “country club” buildings for its exclusivity and amenities.  Opened in 2012, it’s free for tenants and their employees.