The Spare Cash Ledger: Overpayment, Offset or Savings?
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When a household has built a cash cushion, a familiar question follows: should the money be used to reduce the mortgage, held in an offset facility, or kept in a separate savings account? The rule of thumb that says overpaying is always best is too blunt for most families. The arithmetic is rarely difficult, but it does require a few honest inputs.
Begin with the rate you actually pay
The starting point is the mortgage rate, not the advertised rate on a savings account. A borrower on a fixed rate of 4.5% who holds cash earning 4% gross is not comparing like with like. Interest on savings may be taxed, while the interest saved by reducing a mortgage balance is not. An offset account sits between the two: the linked savings reduce the mortgage interest charge without the money being paid to the lender as capital.
The offset rate also matters. Some offset mortgages carry a slightly higher rate than comparable non-offset deals, and some charge a fee. A facility that costs an extra 0.2% a year is still useful if it preserves access to cash, but the cost belongs in the comparison.
Three destinations for the same pound
An overpayment reduces the capital balance permanently. It lowers the interest charged each month and shortens the term if the monthly payment stays the same. The money is no longer available for emergencies, school fees or a new boiler unless the lender agrees to release it. Most fixed-rate mortgages allow an annual overpayment of 10% of the outstanding balance without charge, but the allowance is not unlimited.
An offset account keeps the cash in a savings facility linked to the mortgage. The lender charges interest on the net balance, so £20,000 in the offset reduces the interest bill as though £20,000 had been repaid, while the £20,000 remains accessible. This is the central attraction for households that value liquidity. The terms vary, so the offset rate, withdrawal rules and any monthly fee should be read alongside the mortgage documents.
A separate savings account is the simplest option. The money earns interest, remains accessible, and is usually protected by the Financial Services Compensation Scheme up to the applicable limit. The disadvantage is that the interest is taxable once it exceeds the personal savings allowance, and the savings rate may be lower than the mortgage rate.
The tax line that changes the answer
Savings interest is treated as income. The personal savings allowance is £1,000 for basic-rate taxpayers, £500 for higher-rate taxpayers and nil for additional-rate taxpayers. Interest held in an ISA is free of UK tax. That difference can decide a close comparison.
Take £20,000 held for a year. At 4% gross, the interest is £800. A basic-rate taxpayer with the full allowance may pay no tax on that interest. A higher-rate taxpayer with no allowance left could pay £400, leaving £400 net. The mortgage saving on £20,000 at 4.5% is £900, and it is not taxed. In that example, overpayment or offset has a clear advantage before charges are considered.
If the savings rate is 5% and the mortgage rate is 4.2%, the position changes. The gross interest is £1,000. A higher-rate taxpayer might pay £400, leaving £600 net, which is still below the £840 mortgage interest saved. An ISA paying 4.8% tax-free would produce £960, more than the mortgage saving. The lesson is not that one option always wins; it is that the tax status of the savings vehicle must be included.
Early repayment charges and the annual allowance
A fixed-rate mortgage often carries an early repayment charge during the fix. Overpaying above the annual allowance can trigger a charge, usually calculated as a percentage of the amount repaid early. A 2% charge on a £5,000 overpayment is £100. That may be recovered within months if the mortgage rate is high, but it is an immediate cost and should not be ignored.
Offset arrangements generally do not trigger an early repayment charge because the capital balance is not being reduced. That makes offset more flexible during a fixed term. Borrowers should confirm whether the offset facility itself has a fee and whether the annual overpayment allowance still applies to any separate capital repayments.
The buffer is not idle money
Before comparing rates, a household should decide how much cash must remain accessible. A common starting point is three to six months of essential spending, with a larger reserve for variable income, self-employment or dependants. Money used for an overpayment cannot usually be recalled quickly. Money in an offset account or a savings account can.
A buffer prevents borrowing at short notice, often at a higher rate than the mortgage. It also avoids the risk of missing a payment after an unexpected repair or a change in income. For many families, the correct answer is to fund the buffer first, then compare the remaining surplus.
A one-page worksheet
A simple table is enough:
- Mortgage balance, rate, remaining fixed term and early repayment charge.
- Annual overpayment allowance already used.
- Offset rate, fees and withdrawal terms.
- Savings rate, tax status and whether the money is in an ISA.
- Net savings return after tax and any account fee.
- Mortgage interest saved on the proposed amount over twelve months.
- Any early repayment charge or arrangement fee.
- Required accessible buffer.
- The amount left for comparison.
Run the figures over the remaining fixed term as well as over one year. A small rate advantage can be overtaken by a fee or a charge if the money is moved only briefly.
When each option tends to win
Overpayment tends to win when the mortgage rate is clearly above the net savings rate, the buffer is already funded and no early repayment charge applies. Offset tends to win when the rates are close, the household values access to cash and the offset rate is not materially higher. Separate savings tend to win when the net savings rate is higher than the mortgage rate, when the money is needed for a near-term goal, or when an ISA allowance can be used tax-efficiently.
None of these choices is irreversible in every case, but an overpayment is the least reversible. That is why the buffer and the early repayment charge belong in the same worksheet as the interest rates.
This article is general information, not regulated financial advice. A mortgage adviser or tax adviser can confirm the figures for your own circumstances.
