Bilzin Sumberg Baena Price & Axelrod has signed a 10-year, $58 million lease for 115,000 square feet of space at Brickell Financial Centre, the first major lease at the building under construction.

The lease signed Tuesday will expand the law firm’s space by 35,000 square feet and leave a big hole at its current home in the Wachovia Financial Center as tenants jockey for space in Miami’s biggest office construction boom in two decades.

John Sumberg, the firm’s managing partner, said he met with the developers of three office towers under construction in downtown Miami and the Brickell financial district before opting for the Foram Group’s green project.

The building’s core and shell have been pre-certified at the silver level by the U.S. Green Building Council. Loretta Cockrum, Foram chairwoman and chief executive officer, said the developer is seeking a more stringent gold certification.

The law firm has an option to extend its lease up to 25 years. Sumberg and Cockrum declined to specify any other details of the deal.

Sumberg said the firm’s reasons for choosing Brickell Financial Centre include the green element, proximity to Mary Brickell Village and downtown, I-95 access and Foram’s plans to develop, manage and occupy the building.

The firm is expected to move in January 2011 after its lease expires on 80,000 square feet of space at the downtown Wachovia building.

The new Brickell building will offer 1.5 million square feet in two phases. The project includes more than 1 million square feet of Class A office space, a five-star hotel, 30,000 square feet of retail space and a 30,000 square-foot plaza.

The 601,000-square-foot first phase is expected to cost $150 million to build, or $250 per square foot, Cockrum said.

The other two new office projects are the downtown Met 2 being built a block north of the Miami River and 1450 Brickell.

The big-name lease provides cache to the firm’s future home in a market that will turn from tight to loose as the new projects open.

Bilzin is one of six major downtown and Brickell Avenue tenants with leases expiring in time to take space in the new towers, which will add 1.8 million square feet to the area’s office inventory.

Others include Greenberg Traurig, Hunton & Williams, Bank of America, Deloitte & Touche, and UBS.

Sources say Greenberg Traurig has a “handshake deal” with the developers of Met 2 to lease for less than $40 a square foot, well below the downtown average, where Class A rents range from the low $40s to $50 a square foot.

At $50.43 a square foot including operating costs, the Bilzin lease is near the high end of the current price range. Sumberg said price wasn’t the most important factor.

“Price is always a part of it,” he said. “Price is a significant factor, and we think we got a good deal. The price had to be competitive, and it is. The other issues including the quality and the location, the ingress and egress were also significant factors. If the price had been very much higher, we wouldn’t have been able to do it.”

Foram isn’t looking to snag tenants by offering the lowest prices but by offering high-end space made with high-quality materials and extensive amenities, Cockrum said.

“They could have had a lower price at the other properties,” she said. “We would never compete on price. We have a different cost structure. I have to come back to saying Brickell Financial Centre is not for everyone.”