Developer Hobbs Brook Management is confident in the demand for big blocks of new office and lab space in the suburbs and is working on a 500,000-squarefoot spec building in Waltham scheduled for completion in 2021.

The largest class A office building along Route 128 will go up at the hands of Gilbane Building Co. workers.

Developer Hobbs Brook Management announced the selection Tuesday morning. Gilbane will provide preconstruction and construction management services for the 500,000-square foot speculative building at 225 Wyman St. in Waltham’s Hobbs Brook Office Park. The building will offer the largest contiguous block of class A space in the area.

Construction on the 5-story, lab-capable building designed by Gensler will begin in August and be completed in 2021. The project will also include a new parking garage within the Hobbs Brook Office Park to accommodate parking needs for the complex. JLL is the exclusive leasing agent.

“We selected Gilbane for their extensive expertise in planning and building laboratory facilities,” Hobbs Brook Management principal architect Brad Cardoso said in a statement. “Gilbane has previously worked with Gensler on projects of this size and they are well-positioned for a successful project.”