Offset mortgages explained: when linking savings can help

An offset mortgage links a savings account to your mortgage, and you are charged interest only on the difference between the two. Hold 30,000 pounds in the linked account against a 200,000 pound mortgage and you pay interest as though you owed 170,000. The savings earn no interest, but they save you mortgage interest instead, which is normally a better deal because that saving is not taxed and mortgage rates are usually higher than savings rates. You keep full access to the money.

Your lender takes the balance of the linked account or accounts each day and subtracts it from the mortgage balance before calculating interest. The offset is usually calculated daily and applied monthly, so money sitting there for a fortnight still helps.

Crucially the savings are not used to repay the mortgage. They remain yours, in your account, available to withdraw at any time. Withdraw them and the offset benefit simply stops for as long as they are gone. That is what separates an offset from an overpayment. An overpayment is one way, and getting it back means asking the lender and hoping they agree. An offset is reversible by making a transfer.

Many offsets allow several linked accounts, so you can include a partner's savings, a business tax reserve, or a pot earmarked for a future purchase. Some allow a linked current account. A few family offset products let a parent's savings offset a child's mortgage without giving the money away, which can help a first time buyer without the parent losing control of the capital.

This is the choice that decides whether an offset saves you money or gives you breathing room, and lenders usually let you pick.

Payment reduction means each month your payment is recalculated based on the offset balance. Your monthly outgoing falls while savings are linked, and the mortgage still ends on the original date. This suits you if cash flow is the point: an irregular income, a period of reduced hours, school fees.

Term reduction means your payment stays the same as it would have been without the offset. The extra goes to capital, so the balance falls faster and the mortgage finishes early. This is where the real saving sits, and it is normally the better choice if you can afford the full payment.

Some lenders default you to one or the other, so check and, if necessary, ask them to change it. It is a five minute conversation that can be worth thousands.

This is the part most comparisons miss. Interest on ordinary savings is taxable income. The personal savings allowance lets basic rate taxpayers earn a set amount of savings interest tax free each year, with a smaller allowance for higher rate taxpayers and none at all for additional rate taxpayers. The allowances can change, so check the current figures.

Offset savings earn no interest at all, so there is nothing to tax. You are effectively getting a tax-free return equal to your mortgage rate.

Compare the two properly by grossing up. If you are a higher rate taxpayer paying 40% and you have used up your allowance, a savings account paying 4.5% leaves you with 2.7% after tax. An offset against a mortgage at an assumed 4.5%, purely as an illustration, is worth the full 4.5%. To match that you would need a savings account paying 7.5% gross. Very few do.

The gap widens the higher your tax rate and the larger your savings. For an additional rate taxpayer with a substantial cash balance, the offset comparison is rarely close.

Assume a 250,000 pound repayment mortgage over 25 years at an offset rate of 4.7%. Treat that rate as an illustration, not a market quote.

Without any offset, the monthly payment is roughly 1,414 pounds and the total interest over the term is about 174,000 pounds.

Now suppose you keep an average of 40,000 pounds in the linked account throughout, and you choose term reduction. Interest is charged on 210,000 rather than 250,000, but you keep paying 1,414 pounds. In the first month alone, interest charged falls by roughly 157 pounds, and all of that goes to capital instead. Compounded over the years, this typically clears the mortgage several years early and saves a large five figure sum in interest, with the exact figure depending on how the balance moves over time.

If instead you choose payment reduction, your monthly payment drops by roughly 157 pounds in that first month and the term stays at 25 years. You have converted the same benefit into cash flow rather than into an earlier finish.

Either way, your 40,000 pounds is still your 40,000 pounds. That is the point.

Offsets suit people with meaningful cash balances relative to the mortgage. As a rough guide, if your savings are less than about 5% of the mortgage the benefit may not cover any rate premium, though this depends entirely on the pricing available.

They suit the self-employed particularly well. Money set aside for a January tax bill can sit in an offset for most of the year, doing useful work, and then be paid to HMRC on time. The same applies to VAT reserves and to anyone holding a large emergency fund they refuse to lock away.

They suit higher and additional rate taxpayers, for the tax reasons above. They suit people who want to overpay but are nervous about losing access to the money. And they suit borrowers with irregular income who value the ability to lower the payment in a lean month.

They suit you less well if you have little spare cash, if you would rather chase the lowest headline rate, or if your savings are already sheltered in ISAs earning a competitive tax-free return, where the comparison is closer.

Offset products are sometimes priced slightly above the equivalent standard product, and the range of lenders offering them is narrower. Do the arithmetic rather than assuming. If the offset rate is a quarter of a point higher, work out whether your average linked balance more than covers that.

Money in the offset is not protected from your own decisions. It is easy to spend savings that are sitting in an accessible account, which quietly removes the benefit.

Offset savings still sit with the same institution as your mortgage, so consider how that fits with deposit protection limits if the balance is large.

And an offset is not a way to make an unaffordable mortgage affordable. Lenders assess affordability on the full loan, and rightly so.

Ask how frequently the offset is calculated, whether multiple accounts can be linked, whether you can switch between payment reduction and term reduction later, and what happens to the offset arrangement when the product period ends. Ask whether there is an early repayment charge, because offsets are still ordinary mortgages in that respect.

Because only a subset of lenders offer offsets and their terms vary, it is worth finding an FCA-authorised broker through the MortgageMatch directory who can compare the offset range against a standard product for your actual savings balance.