Setting up one of two new unitary councils in Dorset could cost at least £400,000, councillors have been warned.
The extra money will be needed when Dorset County, East Dorset, North Dorset, Purbeck, Weymouth & Portland and West Dorset councils merge in April, they heard.
Shadow council members were told the extra cash was the 'best guess' regarding the cost of paying for temporary staff who would be needed because currently serving officers are already maintaining services in the existing councils.
Keith Cheeseman, the officer in charge of the process, said the bill 'could go down as well as up' because unless some officers could be released to work on the transformation project he would be forced to hire temporary staff.
It has already been estimated that redundancy costs for four chief executives set to lose their posts in the merger could reach £1m.
The county's other authorities, Bournemouth, Poole and Christchurch, will form a second council under the plans which supporters say could save £108m over six years.
Christchurch Borough Council, which initially opposed the plans, was refused permission to appeal against a High Court ruling over the merger in August.