Reclusive billionaire developer Sheldon Solow filed plans to build a 42-story mixed-use tower adjacent to his long-vacant development site along the East River, records show.

The owner of one of the city’s most expensive office buildings, 9 West 57, and a portfolio of East Side rental towers, Solow submitted plans to build a 700,000-square-foot apartment tower with 550 units at 685 First Avenue, according to an application filed with the Department of Buildings Friday.

The site, currently home to a parking lot, sits across First Avenue from a six-acre lot Solow bought from Con Edison in 2005 for  $109 million.

Plans call for roughly 10,000 square feet of commercial space, including retail, and another 56,000 for manufacturing.

Solow owns a number of sites along the East River that had figured into his $4 billion redevelopment plan for the area. He’s sold off portions in recent years.

In 2010, Solow sold a site at the northeast corner of First Avenue and 35th Street to the city’s School Construction Authority for $33.25 million.

He also sold the remaining portion of that full block in 2013 for $172 million to JDS Development, which is constructing the 800-unit 626 First Avenue on the property.