Offset mortgages represent one of the biggest mortgage innovations seen in recent years. Six years ago there was hardly an offset mortgage to be seen. Now they and the current account mortgage, to which they are closely related, account for £10 out of every £100 of new lending.

What’s more, one of the UK’s large lenders believes that 25% of existing mortgage holders would be better off with an offset mortgage. So if you’re in the market for a mortgage you need to know what they’re all about. Otherwise you could be missing out.

Firstly, how does an offset mortgage work?

The basic idea is that besides borrowing money from the mortgage lender, you also run savings or deposit accounts with them. Then you are charged interest not simply on what you have borrowed but on what you have borrowed less the balance in your savings and deposit accounts. So, if you had an offset mortgage of £100,000 and had £20,000 in their savings account you would only be charged interest on the difference, £80,000. In these circumstances, no interest is paid on your savings – the interest is offset.

It doesn’t sound like a ground breaking idea – where’s the benefit?

Quite simple. Whilst the full benefit of your savings is reflected in a lower interest charge on your mortgage account, legally you have not received any interest. If you have not received interest you can’t be charged tax on the interest. Step away Mr Taxman!

This means that offset mortgages are especially attractive for higher rate taxpayers who would otherwise pay-away 40% of the interest they receive in tax.

Consider some figures. If you had a £100,000 mortgage paying a competitive rate of 4.69% plus £20,000 on deposit, how would the figures work out? Well over a typical 25 year mortgage, without offset you would pay £85,351 in interest but with offset you would pay just £41,998 – that’s a saving of £43,353. What’s more you would repay the mortgage five years and eight months early. That’s because the monthly repayments are based on the full mortgage debt before offsetting is taken into account so borrowers are effectively overpaying their debt each month.

And doesn’t Mr Taxman look sorry! In theory, a standard tax payer saved £9,538 in tax and a higher rate taxpayer a whopping £17,341 in tax.

Flexibility can also be a major advantage. You can typically pay off capital without penalty, underpay and take payment holidays so long as you’ve made sufficient overpayments throughout the years.

Too good to be true – where’s the catch?

Historically borrowers have had to pay a higher interest rate for the benefit of an offset mortgage. But the good news is that with banks and building societies fighting for a bigger share of the offset market, offset interest rates are falling.

This means that you need to look carefully to ensure that the apparent tax savings you could make are not eliminated by the slightly higher interest charge. Quite honestly this is not an easy calculation so it’s best left to your professional mortgage adviser.

But as a guide, a standard taxpayer needs around £20,000 in savings behind a £100,000 mortgage to make the offset deal better value than a traditional mortgage. For a higher rate taxpayer the savings requirement drops to around £10,000. (These figures are based on a typical 4.69% fixed offset rate, compared with a typical 4.49% rate for a tracker.) These figures will change as interest rates vary and, in all probability, as the cost differential between an offset and a traditional mortgage closes.

Not all Offset Mortgages are the same!

As you would expect, with the offset lenders fighting for your business lots have added bell and whistles to the basic concept. Free property valuations and free legal work are relatively common. Then some banks will include your current account in the offset calculation, some lenders enable two nominated savings accounts to be offset, some will even agree an additional borrowing facility with a cheque book that can be used at any time.

On the interest rate front you’re bound to be offered a low starting rate fixed for six or twelve months. You might also be offered a tracker which is below the Bank of England base rate for six months and which only rises above after six months or a tracker which exactly tracks base rate plus a tiny premium for a few years. There are lots of variations.

The interest rate can also depend on what percentage of the house valuation you want to borrow. For example, one lender is currently offering 5.6% if you are borrowing less than 50% rising to 6.45% for up to 99%.

Like so many things, whilst the basic concept is simple, it then gets complicated! This clearly underlines the need to talk things through with an independent mortgage adviser. It’s their job to ensure you get the right type of mortgage and the best deal.

If you have savings, there’s a big chance they’ll recommend an offset mortgage.

*Indicative figures correct as at November 2005

Michael Challiner has 15 years experience in financial services marketing at senior level.

Michael now works as the editor of http://www.kings-college-brokers.co.uk