Homeowners are being urged to check their council tax band to see if they are entitled to claim money back from their local authority.
Have you been paying too much?
Properties were put into council tax bands in 1991, and many valuations were wrong, said consumer campaigner Martin Lewis of MoneySavingExpert.com
He said those who have been overpaying could be in line for a windfall.
Homes in Wales were revalued in 2005 and revaluation is planned in England and Scotland later.
Hundreds of people have reclaimed cash after discovering their homes have been banded incorrectly, Mr Lewis said.
He added many of the 1991 valuations made were "second-gear" valuations.
"These didn't involve someone coming to your home," he said.
ARE YOU ENTITLED TO A WINDFALL?
1. Compare banding with your neighbours. Visit www.voa.gov.uk (England and Wales) or www.saa.gov.uk (Scotland)
2. Work out your house price at 1991 levels through www.moneysavingexpert.com/council
3. If you think you are in the wrong band contact your local listing officer through the www.voa.gov.uk website
"They are so called because estate agents would literally drive past houses and allocate a band."
Mike Linsky, a London taxi driver living in Milton Keynes, received £2,600 back after challenging the banding of his home.
"People in the back of the cab say that council tax is the biggest bill they have and I tell them they should try and challenge it," he said.
The process in England has been postponed while the Lyons review into government spending is completed, but reforms of the system are expected to be announced later this year.
After revaluation in Wales, government figures show that there have been more than 18,000 formal appeals made about the new valuation - of which more than 10,000 resulted in an amendment.
Earlier this week a report suggested the average council tax bill would rise by 3.5% in April.