Dan Gilbert's team has had serious conversations within the last two months about a building large addition behind its One Campus Martius headquarters building downtown, Crain's has learned.

One source said the addition, if built, would be about 250,000 square feet behind the 1 million-square-foot building originally built for Peter Karmanos' Compuware Corp. Discussions about adding to the building have taken place in the past, including under Compuware ownership, but this appears to be a fresh round of talks.

Two other sources with knowledge of the talks confirmed they have taken place recently.

A spokeswoman for Gilbert's Bedrock LLC declined comment Wednesday afternoon.

"We cannot comment on rumors and speculation," said Whitney Eichinger, director of communications for Gilbert's Detroit-based real estate company.

One Campus Martius, located at 1050 Woodward Ave., has an almost "W"-shaped design, spread across 15 stories of office and retail space, although there is a northern section that is just three stories tall that could potentially accomodate more space.

For months Gilbert, the founder and chairman of Quicken Loans Inc. and Rock Ventures LLC, has talked about the need for new office space downtown, where he is arguably the central business district's most powerful and influential landlord. While there are vacant buildings, there are few large blocks of contiguous space available.

It's not known what would happen to the two massive murals that Bedrock curated over the last two years on the northern sides of One Campus Martius.

The Shepard Fairey mural was completed in May 2015; the artists How and Nosm (brothers Raoul and Davide Perre) completed the other mural in April.

Gilbert, also the owner of the Cleveland Cavaliers, has been the most active real estate developer in downtown Detroit the last several years.

Most recently last month, Bedrock unveiled its long-anticipated plans for a new high-rise on the site of the former J.L. Hudson's department store site at Woodward and East Grand River avenues. As currently conceived, that building would be 1.2 million square feet with a 734-foot residential tower, which would be the tallest building in the city, besting the Detroit Marriott at the Renaissance Center by 7 feet. The redevelopment would include 279,000 square feet of office space spread out over four floors of a nine-story podium.

Among the other projects Gilbert, who Forbes values at $5.8 billion, is pushing:

A two-block development east of One Campus Martius consisting of an office tower with at least 20 stories and a residential building with at least 16 stories. The area is bounded by Randolph Street, Bates Street, Cadillac Square and Monroe Avenue.

A 10-acre mixed-use development with General Motors directly east of the Renaissance Center. The area is almost entirely surface parking currently.

A $1 billion development with a Major League Soccer stadium and three 18- to 28-story towers for office, residential and hotel uses on the site of the half-built Wayne County Consolidated Jail on Gratiot Avenue with Detroit Pistons owner Tom Gores.

He is also working on redevelopments of the Book Tower on Washington Boulevard, the David Stott Building on Griswold Street, a new 13-story building with so-called micro-apartments and other projects.

In Lansing, Gilbert and a statewide coalition known as MI Thrive is pressing legislation that would allow developers to capture a portion of sales and income taxes to help fund so-called "transformational" developments in cities both large and small. The private investment threshold for Detroit is $500 million. The Michigan Senate passed the legislation last month.

One Campus Martius, constructed in 2003, is owned by a 50-50 joint-venture between Bedrock and Detroit-based Meridian Health, which purchased it from Compuware at the end of 2014 for $142 million.

Gilbert's companies own more than 95 properties — buildings and parking decks, for example — in and around downtown. Those properties total more than 15 million square feet, Bedrock says.