Boston University purchased three streets off of Commonwealth Avenue for $11.5 million as part of a plan to build a pedestrian mall and expand academic buildings, officials said.

BU bought Cummington Street, Hinsdale Street and Blandford Street from the city of Boston, said BU spokesman Colin Riley. The pedestrian mall will be built in the area behind Warren Towers, and the other streets will be closed to make room for possible building expansions.

The streets are now under BU’s ownership, but will not become pedestrian walkways until students return in the fall.

The purchase is part of a long-standing plan to improve the life sciences and engineering research facilities, Riley said.

“This has been planned since our first master plan in 1986,” Riley said. “We always anticipated giving those streets pedestrian access.”

The streets will include additional brick walkways, and the parking meters will be removed, Riley said.

“The key thing is that by owning the streets, we can now build over them,” he said.

Although these streets are privatized, Riley said, they will be open to public pedestrians, as well as emergency vehicles and vehicles delivering supplies to buildings in the area.

BU has plans to build over the end of Cummington Street to make room for the life sciences and engineering edifices, Riley said. While BU has a model of the building, a design has not been created.

If all goes well, plans for the building will be included in the master plan that will be completed in October.

The BU community task force worked with the Boston Redevelopment Authority to take over the three streets, Riley said.

“We’ve been occupied with a lot of projects, but the time had come for this proposal to move forward,” Riley said, “and we’re happy to have it completed and to have come to an agreement with the city.”