The cost of a new art gallery on the Kent coast has reached £29.5m - after originally being estimated at £7m.
The gallery is due to be completed before the end of 2007
Building work on the Turner Contemporary in Margate began in August and is due to be completed in 2007.
The £7m estimate was in 2001 and last year when the design was finalised for the gallery - being built in the sea - it was estimated it would cost £25m.
But the cost is now put at £29.5m, with councillors warning it could eventually be even higher.
The gallery has been designed by Norwegian architects Snohetta and Spence and will house works by JMW Turner and pieces by contemporary artists.
Among those could be Turner Prize winner Tracey Emin, who is from Margate.
Clive Hart, Labour county councillor for Margate and Cliftonville, said: "I desperately want to see this area regenerated. However I have to say the latest rise in the estimated costs of the project cause me great concern.
"Most of the costs seem to be in relation to the unique steel design and the siting of the gallery in the sea.
"What really worries me is that eventually we'll run to a final cost that will render the final project totally unjustified.
"I sincerely hope it works but we should be careful with ratepayers' money."
Conservative councillor Mike Hill, county council cabinet member for community services, said a less ambitious building would not have attracted outside investment.
He said: "We had an international architecture competition to try and find the best building we could and we had an expert panel which advised us.
"We've already attracted £10m worth of external investment in this project and one of the main reasons we've got that is because of this unique and exciting building.
"People like the Arts Council and Seeda (South East England Development Agency) would not have put their money into a boring building, that's for sure."