First Republic Bank is almost doubling its space at Rockefeller Center to occupy 280,000 square feet across two buildings.

The service-oriented financial tenant is expanding by 100,000 square feet at 10 Rock by adding the third and fourth floors of the building, which sits between East 48th and East 49th streets, overlooking NBC’s morning concerts.

Those floors are occupied by a Rockefeller wealth management firm and the medical providers EHE (Engaging Healthy Employees). Both entities will relocate within the Rockefeller complex.

The slender building is adjacent to the Simon & Schuster building at 1230 Sixth Ave. (pictured), where it will also add another 30,000 feet on the eighth floor. It already occupies 150,000 square feet on the second and third floors along with the seventh floor, which it leased in 2014 in an early renewal and expansion when area asking rents were in the $60s per square foot.

Now, area rents can double and nearly triple that amount, reaching as much as $175 per square foot.

The bank’s ground floor branch on Sixth Avenue is known for providing customers with free umbrellas during rainstorms.

Frank Doyle, David Kleiner and Betsy Buckley of JLL represented the bank.

Tishman Speyer Properties, which owns and manages the Rockefeller Center building, declined to comment through a spokeswoman, as did the brokerage.

Despite others moving to Hudson Yards, in the last few years, large leases along the Sixth Avenue corridor have included those by Bank of America, News Corp. (owner of The Post), Twenty-first Century Fox, Latham & Watkins and Major League Baseball.