Kent Online reveals that the cost of building the new Turner Centre has risen by £2.4million to £17.4million.
Kent County Council is confident the price for the new "on-shore" gallery on Margate seafront will not go up any further but admitted it had abandoned hope of imposing a £15million ceiling on the costs.
One of the reasons for the increase is the discovery that the new site could be vulnerable to sea flooding, meaning that additional protection will be needed.
Cllr Mike Hill (Con), KCC's cabinet member for communities, said: "The £15million was an estimate made when we did not even have architects appointed.
"When they were appointed, we ended up with extras relating specifically to the condition of the site and the need to make it 'fit for purpose'. There is an inherent risk of the sea over-topping the sea wall which is minimal but nevertheless means the building requires protection."
The initial commitment to pay no more than £15million also excluded the costs of fixtures and fittings.
The initial plan for the Turner centre on the pier was scrapped last February after costs nearly doubled to £40million. About £7million of public money was spent and KCC has confirmed it is considering legal action to recover some of its outlay