About a million householders may be paying too much council tax, while a similar number of valuations are too low, according to a report by MPs.
The Commons public accounts committee is calling on the Inland Revenue to correct the errors.
Its report last week follows the anouncement that nearly 200,000 council tax appeals have yet to be decided, 15 months after the deadline ended for them to be submitted.
At the same time, around 650,000 householders have won reductions in their council tax-band valuations after appealing - a success rate of 80 per cent.
Their victory follows Government pressure on valuers to clear the backlog, leading to these cases being settled without a formal hearing. In most of them, valuers have given in to applicants and moved properties down by one band, an average reduction of £60 a year.
One million appeals were lodged across England, Scotland and Wales - Northern Ireland still has rates - but 20 per cent of applicants withdrew after realising they had no prospect of success.
The most contentious cases - often where home owners refuse a compromise, or where applicants believe the valuation is more than one band out - have been left until last.
The Inland Revenue, which is in charge of the Valuation Office Agency, does not disclose how many cases go to formal hearing.
But according to figures provided by the Scottish Office, residents are seldom successful when they push their cases and decline compromise. Of 2,095 cases heard by valuation committees in Scotland, just 191 have been allowed, with the rest dismissed.
Some residents who have submitted appeals have still have not been given either an informal hearing or a date for a tribunal.
Joyce Woodward and her husband live in Firsdown, near Salisbury, in Wiltshire. The couple appealed in May 1993 against the banding on their semi-detached bungalow, but are still waiting for a decision.
The original valuation placed their property in band D, which means they have to pay £508.56 a year. They believe the bungalow should be in band C, paying £56.51 less.
"We live in a cul-de-sac and everyone around us is in band C," says Mrs Woodward. "Yet we live in identical bungalows. We can't see why we are different. We have written several letters complaining about the delay in dealing with it, and they have not even answered the last two. All we have been told is that it will probably go to tribunal."
The Government has said that "all but a handful" of cases will be completed by the summer. It adds that new appeals should be handled within a year of their being made.
For those considering an appeal now, the options are limited. A fresh appeal can only usually be made if the value of a property is reduced by a material change of circumstance. A new road at the bottom of the garden, demolition of part of the property, or subsidence may all be valid reasons for claiming a reduction in your council tax valuation.
However, falling property values are not considered grounds for appeal, and properties are valued according to April 1991 prices. At the same time, owners considering moving may want an upward revaluation to assist them in making the case for a higher asking price.
New extensions and adaptations will only lead to an increased council tax valuation from the date of the sale of the property.
For those who have just moved into a property, it is possible to appeal within six months of the move, even if the previous occupant had not appealed.
Sue Waller, a housing adviser with the Camden Federation of Private Tenants, said this option was little known, but very useful. She said: "Tenants can appeal against listing, for example, to say that it is a bedsit and that the landlord is liable for the council tax. A new resident can also appeal against the banding."
Residents of newly built properties may find it takes some time to establish the extent of their council tax liability. The Valuation Office is required to produce an assessment within three months of a building first being occupied, but this requirement is not always met.Reuse content