Medical staffing firm Jackson Healthcare says it will start work within months on a $100 million expansion of its Alpharetta headquarters campus.

The company, which operates nationally, plans to add 306,000 square feet to its headquarters, enough to handle 1,400 more employees, the company said.

Specific hiring plans were not detailed in the company’s announcement. Jackson now has about 1,100 employees, 898 of them in metro Atlanta.

The expansion, which will take about 18 months to complete, will produce a campus “reminiscent of the Italian Renaissance,” the company said. It will include an eight-story, 267,000 square foot office building and a separate “amenities” building inspired by the Roman Colosseum. That second, 39,000 square-foot structure will include a gym, an indoor pool, a barber shop, a salon, childcare facilities, a tanning salon and a restaurant.

The architect is Rule Joy Trammell + Rubio, LLC Architecture + Interior Design, which designed a piazza meant to evoke Italy, officials said.

Jackson, which is privately held, had annual revenue of $798 million in 2015. The company is parent to a number of subsidiaries, has offices in seven states and places healthcare workers around the nation.

Many employees at the expanded Alpharetta campus will be tech workers and sales staff, the company said

Jackson and the city negotiated an economic incentive package as part of the planned expansion, but calls Friday to the Alpharetta Community Development Department were not returned.