OAKLAND -- Bay Area regional transportation planning agencies' move into a 1942 former post office in downtown San Francisco will cost $48 million more than initially estimated.

Increased seismic retrofit costs, internal improvements and $15 million worth of furniture, fixtures and equipment for 390 Main St. will drive up the final price tag nearly 30 percent to $215 million.

The Bay Area Toll Authority, county and local elected officials who oversee bridge tolls, approved the budget increase Wednesday.

"It's never a good thing to have your construction budget go up, but this is justified," said Steve Kinsey, an authority member and Marin County supervisor. "In my experience with these large projects, once you pull the trigger, you have to move forward."

The authority and the related Metropolitan Transportation Commission, Bay Area Air Quality Management District and the Association of Bay Area Governments intend to co-locate in the eight-story reinforced concrete building after renovations are finished and will share in the costs.

The commission's vote to move out of Oakland triggered massive opposition from East Bay elected officials and businesses. State Sen. Mark DeSaulnier, D-Concord, who is chairman of the Senate Transportation Committee, repeatedly but unsuccessfully challenged the authority's use of toll dollars to finance the building purchase and renovations.

And as critics warned during heated public hearings at the time, the $1 million budgeted for seismic retrofit proved far short of the $11 million that staff now estimates it will cost.

"We obtained two seismic estimates and, unfortunately, they were both wrong," MTC Executive Director Steve Heminger told the toll authority.

Other added costs include $3 million to construct an atrium in the center of the building, which staff says will bring essential light into the old warehouse and help make it more attractive for the other tenants of the building. Roughly one-quarter of the building will be rented to private companies.

Contact Lisa Vorderbrueggen at 925-945-4773, politicswithlisav.blogspot.com or Twitter.com/lvorderbrueggen.