A plan to wrap the lower three levels of Chicago’s Willis Tower with an addition containing new retail, dining, and entertainment space has taken an important first step. Late last week, the City of Chicago issued a renovation permit for interior demolition and structural work for the Gensler- designed annex. The project is part of a larger $500 million overhaul of the 1,451-foot-tall skyscraper revealed earlier this year by property owner Blackstone Group and partner Equity Office.

While some upgrades to the tower such as an updated elevator system and the creation of a new 33rd floor fitness center have been ongoing, the reimagining of the tower’s existing—and rather fortress-like—base will be the biggest and most visible phase of the project thus far. Estimated at $50 million, the newly-issued permit outlines the following improvements:

Demolition of lower levels (ll), ll1, ll2, and ll3 and upper levels (l) l1, l2, and l3. Scope to also include new structural addition to base of the building. Superstructure addition and interior demo only for the lower levels. Full build-out permit to follow.

The 300,000-square-foot addition is expected to include an undulating glass atrium and high-end restaurants, casual eateries, and a fresh food market designed to serve both tenants of the building as well as the greater neighborhood. The new structure’s roof will feature 30,000 square feet of publicly accessible open space and could host a wide variety of programming including ice skating in the winter months.

The project will see the entrance and assembly area for the tower’s 103rd-floor “Sky Deck” streamlined and overhauled. More ambitious future upgrades to the observatory could include expanding the space to a second level and adding new “thrill attractions” such as a chance to rappel down the side of the building inside a transparent enclosure.