The £100,000 Cliff Edge: Planning Around the Personal Allowance Taper in 2026/27

Most taxpayers think of Income Tax as a set of rates: 20%, 40% and 45%. The personal allowance taper is different. It does not appear as a separate rate on the tax tables, yet it can make the income between £100,000 and £125,140 some of the most expensive a person ever earns.

For 2026/27, the standard personal allowance remains £12,570. Once adjusted net income passes £100,000, the allowance is reduced by £1 for every £2 of income above that threshold. By £125,140, the allowance has gone entirely. The result is a marginal rate of 60% for many people in that band: 40% Income Tax on the extra income, plus the loss of allowance that would otherwise have sheltered income at 40%.

How the taper works in practice

Take a simple employee with £110,000 of salary and no other income. Adjusted net income is £110,000. The personal allowance is reduced by £5,000 (half of the £10,000 excess), leaving £7,570. Taxable income is therefore £102,430.

The £10,000 above £100,000 is not taxed at a flat 60% in the technical sense. But the combined effect of higher-rate tax and allowance withdrawal means the effective marginal rate is 60% across that slice. At £125,140 the allowance reaches zero, and income above that point is taxed at the additional rate, subject to the usual rules.

This matters because the threshold is not index-linked in the way many people assume. A bonus, a pay rise, savings interest, dividend income or a good year in self-employment can all push adjusted net income over £100,000. The tax return then produces a higher bill than the headline rates might suggest. Scottish Income Tax rates differ, but the personal allowance taper is UK-wide, so the effective marginal rate for Scottish taxpayers can differ.

Adjusted net income is not taxable income

The taper is based on adjusted net income, not the figure shown as taxable income after allowances. Adjusted net income starts with total taxable income and subtracts certain reliefs, including gross pension contributions made under relief-at-source arrangements, gross Gift Aid donations, trading losses and some other reliefs.

That distinction is important. A workplace pension operated on a net pay basis may already have reduced taxable pay, so the starting figure could be lower. Salary sacrifice works differently again because it reduces contractual salary. Relief-at-source personal pensions and Gift Aid donations reduce adjusted net income and can extend the basic-rate band. Checking the right figure is the first practical step.

Using pensions and Gift Aid to reduce the cliff edge

A relief-at-source pension contribution is paid net of basic-rate tax. The provider adds 25% and claims that basic-rate relief from HMRC. For every £8,000 paid net, the gross contribution is £10,000. Higher-rate or additional-rate relief is then claimed through Self Assessment, and the gross contribution also reduces adjusted net income.

Gift Aid works in a similar way. A donation made under Gift Aid is treated as paid net of basic-rate tax. The charity claims the basic-rate top-up, and the gross donation reduces adjusted net income. Higher-rate relief is claimed via the tax return. The donor must have paid enough tax to cover the Gift Aid claimed across all donations.

Suppose the employee with £110,000 of income pays £8,000 net into a relief-at-source pension. The gross contribution is £10,000, so adjusted net income falls to £100,000 and the full personal allowance is restored. The combined value of the relief can be substantial: the provider claims £2,000 in basic-rate relief, and around £4,000 may be recovered through Self Assessment because of the higher-rate relief and the restored allowance. The exact saving depends on the rest of the return.

Practical checks before the 2026/27 return

1. Estimate adjusted net income now. Include salary, bonuses, self-employed profits, rental income, savings interest, dividends and taxable benefits. Then subtract gross pension contributions and Gift Aid donations.
2. Check the pension type. Net pay, relief at source and salary sacrifice affect the figures differently. Payslips and pension statements should show the gross contribution.
3. Time bonus and dividend decisions. If income is close to £100,000, the timing of a bonus or dividend can change the allowance available in 2026/27.
4. Keep Gift Aid records. A valid declaration is needed, and the gross donation must be calculated correctly. Keep bank records and charity confirmations.
5. Do not over-contribute just to beat the threshold. Pension access rules, annual allowance limits and investment risk matter. Relief above the amount needed to restore the allowance follows the normal rules.
6. Remember the filing date. The 2026/27 Self Assessment return is due by 31 January 2028. Payments on account may also be affected if the tax bill rises.

The £100,000 threshold is not a trap in the sense of a hidden penalty. It is a predictable interaction between two parts of the tax system. The danger is ignoring it until the return is prepared. A mid-year check, a careful record of pension and Gift Aid payments, and a realistic estimate of adjusted net income can prevent an unpleasant surprise.