The City Council's proposed redevelopment of the former Deka store in George street could be more expensive than first thought.

The construction budget for the proposed mall has almost doubled to $20 million.

Design and feasibility studies have been underway for several months, with the redevelopment scheduled to start in May this year.

The council's earliest proposal considered adding two storeys for carparking, but an 80% increase in construction costs means the redevelopment is likely to be a lot more.