Developer Mitch Rutter has plans under way to build a 300,000-square-foot tower on the northeast corner of 42nd Street and Fifth Avenue, The Post has learned.

Demolition has just begun on the bazaar of small stores at the corner, while the adjacent buildings, 503, 505 and 507 Fifth Ave., are being cleared of their tenants.

With a new 60,000-square-foot retail base to tantalize strollers passing the public library, retailers are already scrambling for a piece of the building. Even some non-retailers are looking at the location.

“This is a corner for a Fortune 100 company or a specialty store,” said Faith Hope Consolo, vice chairman of Garrick-Aug Store Leasing.

“It is really like buying $100 million worth of advertising, and they will have the signage but are not lost in Times Square.”

Developer Mitch Rutter of Essex Capital Partners declined to comment on the building, which others familiar with the project said “is still evolving.”

He has been approached by hoteliers but is still expected to build an office tower. It will, no matter what, have “world class architecture.”

Already on board is store-design architect Michael LeClere, who was involved in the Warner Brothers Studio Store on Fifth Avenue at 57th Street.

The ultimate retail tenant “may not make any money,” said Jack Chanler of FirstAtlantic Realty Partners. Store rents are expected to be more than $200 per square-foot for the multi-level space.