Investment firm One William Street Capital Management is adding on a bit more space at 299 Park Avenue.

One William Street, which manages investment strategies on asset-based and structured credit opportunities, has tacked an additional 15,000 square feet onto its existing 29,754 square feet, bringing its total footprint at the 44-story office tower to 44,754 square feet, according to landlord Fisher Brothers.

“Investing in the tenant experience is a core principle at Fisher Brothers,” Winston Fisher, partner at Fisher Brothers, said in a statement. “With our recent investments in capital improvements and infrastructure upgrades, we have experienced strong leasing momentum at 299 Park Avenue, yet another signal that leading tenants are continuing to prioritize well-located, trophy-quality office space.”

One William Street signed a 12-year lease for the entire 25th floor of the Midtown East building between East 48th and East 49th streets in October 2018, moving from 1290 Avenue of the Americas, as Commercial Observer previously reported.

The length of the lease and asking rent in the new deal were unclear, but asking rents in the building ranged from the mid-$90s to the mid-$130s per square foot last year.

CBRE’s Scott Gottlieb, Andrew Sussman, Ben Friedland and Lewis Gottlieb brokered the deal for the tenant. Fisher Brothers’ Marc Packman and Clark Briffel represented the landlord in-house along with Newmark’s David Falk, Peter Shimkin, Andrew Sachs and Eric Cagner.

Spokespeople for One William Street, CBRE and Newmark did not immediately respond to requests for comment.

But One William Street isn’t the only tenant making moves at 299 Park Avenue.

Private equity investment firm P10 Intermediate Holdings expanded its office space at the 1.3 million-square-foot office tower by nearly 9,000 square feet, while consulting firm Tailwind Management signed a new lease for nearly 14,000 square feet, according to Fisher Brothers.

It’s unclear when P10 moved into the building and how much space it took, but Tailwind will move to 299 Park Avenue from its previous office at 485 Lexington Avenue.

CBRE’s Silvio Petriello brokered the deal for P10, while Savills’ Greg Taubin represented Tailwind. Packman, Briffel and the Newmark brokers represented the landlord.

Spokespeople for P10, CBRE and Savills did not immediately respond to requests for comment, while a spokesperson for Tailwind could not be reached for comment.

The new deals at 299 Park Avenue come as Fisher Brothers is building five prebuilt office spaces on the third and fourth floors, with three of those spaces officially pre-leased, according to the landlord. Those tenants will have access to a gym and a custom-built lounge.

In addition, in the last few years, Fisher Brothers completed a $20 million capital improvement project at the office tower, including upgrades to the lobby, facade and elevators. And earlier this year, the landlord secured $500 million from Citigroup and J.P. Morgan for a refinancing, according to the release.

Other tenants of 299 Park Avenue include investment advisory firm MacKay Shields, middle market private equity firm Kelso & Company, investment firm Crescent Capital Group, real estate investment company Lightstone and global investment bank Lincoln International.

Isabelle Durso can be reached at idurso@commercialobserver.com.