Blackstone Group LP and affiliate Equity Office plan to modernize the 43-year-old Willis Tower in downtown Chicago.

The private equity giant is allocating $500 million for the project, which Global Head of Real Estate Jon Gray said is Blackstone's largest investment in "reimagining" one of its properties.

The project will involve reconfiguring about 460,000 square feet of existing space at the iconic tower to provide 150,000 square feet of space for tenant use; more than 300,000 square feet of additional retail, dining and entertainment space at the base of the tower; and a new 30,000-square-foot outdoor deck and garden space. The Skydeck Chicago experience on the building's 103rd floor will also be enhanced as part of the investment.

The planned overhaul is expected to kick off in February, according to a release.