Rief said he is planning to invest $2.5 million to $2.7 million on build-out of the space. When it opens, it will be the first Planet Fitness in the downtown area and third in the city, the others of which are on the borders with Ferndale and Dearborn.

Rief is eyeing a May opening for the downtown gym. It will likely have eight full-time and 12 part-time employees.

Four other locations are also planned for metro Detroit this year, and Rief is scouting future locations in the city, including Midtown and New Center.

"After 2017, we're still very bullish in the metro Detroit market," he said. "We're looking to open four to five (new gyms) per year. Downtown Detroit is especially interesting to us and it's an area where we are going to focus our attention."

Livonia-based PF Michigan Group is a franchise group that operates 35 Planet Fitness locations in Michigan, 26 of which are in metro Detroit, and four in Ohio. There are more than 60 locations throughout the state.

The LLC was founded in 2008, when it opened its first location in Livonia, and it has opened several gyms each year since then. Its 2017 revenue, which includes sales from five clubs it acquired last year, was more than $74 million, Rief said. With its average unit volume of more than $2 million per year, its total revenue has increased at a clip of 34 percent per year since 2013.

Rief said leases have been signed, or are in the process of being finalized, for locations in the city of Wayne, downtown off Michigan Avenue; Garden City, at Ford and Middlebelt roads; Auburn Hills, in Great Lakes Crossing Outlets; and Lake Orion, off Lapeer Road.

PF Michigan opened its most recent locations, in Mount Clemens and Milford, in December.

The standard size of a Planet Fitness Gym is 35,000 square feet and each one employs around 20 people. The gym in downtown Detroit will look very similar to other locations in metro Detroit, Rief said.

The contractor for the downtown project is K & M Building Co., based in West Bloomfield. The designer is Chicago-based RGLA Solutions Inc.

Rief said he decided on the downtown location because it fit the criteria for location, size, high ceilings and access, and especially because it has space for dedicated parking. The building is the former headquarters of Crain Communications Inc.

"Once we open the riverfront location (in Detroit), it will demonstrate to some of those developers that we would be a desirable tenant," Rief said.