The Association of National Advertisers is restacking and expanding at 10 Grand Central to around 51,000 square feet.

The ANA has been growing rapidly since it started out in the tower of the building on one of its 4,000-square-foot floors. After it gobbled up all of those, it also leased the 23rd, the second and, most recently, the eighth floors.

Something had to change. “They were all over the building and felt they were missing out on the collaboration they needed, and as a marketing firm, they also wanted to have those ‘casual collisions,’ ” explained Jeffrey Peck, who led the ANA’s Savills Studley brokerage team, which included Daniel Horowitz and Kurt Handschumacher.

The fourth floor became available and, after discussions with the Marx Realty ownership and its JLL brokers, Howard Hersch, Sam Seiler and Cynthia Wasserberger, a decision was made to relocate some of the tenants on the third floor, which had such clauses in their leases.

The ANA will now have 11,908 square feet on the third floor with an option to expand, and an added 20,691 square feet on the fourth floor while occupying the contiguous space on the second floor.

The ownership will also spiff up the exit staircase, allowing folks to run up and down and in and out with key-card access. ANA will also keep its current eighth floor, and all the leases are concurrent. The asking rent was in the $60s a square foot.

Because it recently had a good experience when Marx built out the eighth floor, the owner will also build out the new space.

Under the terms, it will give up the 23rd floor along with the boutique tower floors from 32th through 35th. The latter will be refitted as pre-built space with higher asking rents. Asking rents in the building now are $72 to $97 a square foot.

In the past, the structure, known as the Commerce Building and designed by Ely Jacques Kahn, has had the additional street address of 155 E. 44th St. and was considered a Third Avenue value-play, Peck said.

Now with an approximately $45 million investment including a new hotel-like lobby on its way, “It has been upgraded to the point where it is a Class A quality space offering significant amenities,” Peck said.

Indeed, the ANA’s floors are also in the same elevator bank as the tenant-only seventh floor amenity space that’s “one of the nicest I’ve seen,” Peck said.

Along with a café, there is a board room with advanced technology and a lounge area. There is also a 3,000-square-foot furnished and landscaped outdoor terrace with a firepit and Wi-Fi.