Work started this week to repair the leaking roof of The Cube at Corby.
In September last year borough councillors agreed to spend a further £600,000 on the civic hub to prevent the leak causing damage this winter.
The additional spending brings the total bill for the building, which was to have cost £36m, to £48.2m.
A spokesman for the borough council said: “Contractors have been on site at the Cube since Monday, January 6, and work has now begun on the roof. Completion of the works is expected to take approximately 12 weeks.”
The fourth floor roof of the Cube needs substantial repairs. The area has not been fitted out and its flat roof leaks on to the council chamber, the councillors’ room and offices below.
The council has been involved in negotiations with the Cube design team, Hawkins Brown, and the contractors Galliford Try over who should meet the cost of repairing the leaking roof and rectifying other defects in The Cube.