The downtown real estate empire of Quicken Loans Inc. founder Dan Gilbert may be growing.

Brokers around town are saying that Gilbert's Bedrock Management has One Woodward Ave. under contract to be sold.

Quicken wouldn't confirm the story — Paula Silver, Quicken's vice president for communications, told Crain's "we have nothing to report at this time."

While a closing date isn't known, brokers estimate that the building could sell for $4 million to $6 million.

The 332,000-square-foot building was listed for sale by NAI Farbman. The Detroit Regional Chamber is One Woodward's most prominent tenant; other tenants include the law firms Kitch Drutchas Wagner Valitutti & Sherbrook P.C. and Fraser Trebilcock Davis & Dunlap P.C. and engineering firm CDM.

Gilbert is the second-largest private owner of downtown real estate, behind General Motors. Prior to the reported One Woodward deal, Gilbert owned 1.7 million* square feet of office space in downtown Detroit and roughly 3,500 parking spaces.

Recent Gilbert acquisitions include: the First National Building, the Madison Theatre at 1555 Broadway Ave., the Chase Tower on Woodward Avenue south of Campus Martius, and the Dime Building, renamed Chrysler House.

Controlling downtown parking has been a key element of Gilbert's downtown real estate acquisition plan. He owns most of the parking that serves the Woodward office corridor.

Gilbert's m.o. has been to purchase a building on the cheap, put some money into renovating it, and then move employees from either a Gilbert-owned or Gilbert-affiliated company into the downtown space, a move that's paying off in the form of improving comps for downtown office space.

The latest company to occupy space in a Gilbert-owned building downtown is Sachse Construction, which will move 45 employees into the Arts League building at 1528 Woodward, owned by Gilbert's Rock Ventures. Sachse, a general contractor, built out Quicken's space in the Compuware Corp. building, the Madison Building and the Chase Tower. Sachse is also doing the build out of the space in the Arts League Building, which will be complete in December.

*UPDATE: Quicken Loans' Paula Silver called - she would like me to note that Gilbert actually owns 3 million square feet of commercial space in downtown Detroit, which includes office, retail and parking. Crain's typically doesn't include parking in tallies of total square footage owned.