Millions of people could be overpaying on council tax because of a major flaw in the bands system, Martin Lewis has warned.
The money expert said as many as 400,000 homes are currently trapped on the wrong band, while thousands more people are unaware they qualify for a discount.
In one case, a 63-year-old woman received a £3,500 refund after discovering her property was under the wrong bracket.
In another, a student discovered she was exempt - resulting in a £2,600 refund, while the council cleared her existing balance, too.
"If you have Alzheimer's disease, dementia, have had a stroke or you live alone, you could be due to a discount," Martin Lewis said. © GettyIs your property registered under the right council tax band?
"There are also council-by-council reductions on Covid-19, pensions credit and you could have your band lowered if you have a disability.
"Hundreds of thousands of people are paying the wrong council tax - and that means they could be owed thousands back, Martin added.
The good news is that the money saving expert just explained exactly how to check to see if you could pay less.
And you can get the overpaid tax backdated all the way to 1993 - seeing some serious amounts of cash returned to you.
This is what he said you should check.
1. Check your council tax band
As many as 400,000 homes are understood to be on the wrong council tax band.
If you're in the wrong bracket, and are paying too much council tax as a result, you can backdate your refund all the way to 1993.
And it's simple to check, too.
The first thing to do is check what your neighbours are paying by going to the Valuation Office Agency (VOA) in England or the Scottish Assessors Association (SAA) in Scotland.
It you're paying more than your neighbours are, it's on to step two.
This is where you need to work out what your house was worth in 1991.
The good news is there are plenty of free tools online to let you do this, for example this one from Nationwide.
Once you know that, you can see what band you should have been put in.
But Martin has a warning too.
"Only apply if both check's show you are in too high a band," he said.
That's because if you get it wrong, and it's your neighbours who are in the wrong band, they could see their bills rise, while yours stay the same.
2. Live alone? You could get 25% off
"If you are living alone, you should get a 25% discount on council tax," Martin said.
But he added that you don't need to live alone to claim this.
It's all about qualifying adults, so students, carers, children and others can all be disregarded for council tax.
In fact, in some cases every person living in a house can be excluded - meaning no tax is due at all.
Claims on this can be backdated too.
3. Discounts for benefit claimants
There are discounts of up to 100% for people on certain benefits.
But these aren't automatically applied when you claim the benefit - you need to apply directly through your local council (details of your council is at www.gov.uk/apply-council-tax-reduction).
The reduction comes on top of any benefits or universal credit you receive, but the reduction amount depends on your council and circumstances - for example, where you live, your income, if children live with you, if other adults live with you.
Some councils let you backdate the reduction, but it depends on the individual council, so you need to check.
4. Disability council tax discounts
If you - or someone you live with - has a disability, a health condition or are a carer, you could be overpaying on council tax without even knowing it.
There's a 25% discount if you live with someone with a severe mental impairment and no other adults.
You can also get a discount if you have a severe mental impairment and live alone.
The better news is claims are back-dated - meaning you could get huge amounts back.
5. Coronavirus help
Some also helping out people who have lost income as a result of coronavirus.
The Local Government Association advises residents to check their council website to see who qualifies for a tax break.
It's all thanks to a support fund made available by the Treasury. If you qualify, your council tax could be slashed by £150 a year.
Those who already pay less than £150 a year will not pay council tax at all.
6. Have you adapted your home?
If you've made changes to your home for a disabled resident, you could also get cash back.
But rather than a discount, you may instead be able to get your council tax band lowered.
7. Refunds when you move out
Many people pay council tax a month or a year ahead - and that means if you move house, they keep it until you ask for it back.
So if you've moved in the past few years, search for council tax refunds on your old council's site.