The £100,000 Tax Trap: How Earning £100,001 Could Cost You Over £5,000

Why £100,000 Is the Most Dangerous Number on Your Payslip

If your total compensation has crept above six figures, you need to understand a quirk of the UK tax system that most people discover too late.

I asked AI to analyse the real HMRC numbers for 2026/27, and the results are striking. For every extra pound you earn between £100,000 and £125,140, the government effectively takes 60p — or 62p once National Insurance is included. That is a higher marginal rate than someone earning £1 million pays.

This happens because of the Personal Allowance taper. Let’s walk through it with real numbers.

1. The Personal Allowance Taper — How It Works

For 2026/27, every UK taxpayer receives a Personal Allowance of £12,570 — the amount you can earn before paying any income tax at all. However, once your adjusted net income exceeds £100,000, that allowance is reduced by £1 for every £2 you earn above the threshold.

This means:

• At £100,000 — your Personal Allowance is the full £12,570.
• At £105,000 — you’ve earned £5,000 above the threshold. PA reduced by £2,500, leaving £10,070.
• At £110,000 — PA reduced by £5,000, leaving £7,570.
• At £125,140 — you’ve earned £25,140 above the threshold. PA reduced by the full £12,570. Your Personal Allowance is now £0.

That lost £12,570 of Personal Allowance is now taxed at 40%, costing you £5,028 in extra income tax — on top of the 40% you’re already paying on the income itself.

2. The 60% Effective Marginal Rate Zone

Between £100,000 and £125,140 your effective marginal tax rate is 60% on income tax alone:

• 40% higher-rate income tax on the extra pound earned
• 20% effective rate from the Personal Allowance loss (you lose 50p of allowance per £1 earned, taxed at 40% = 20p)

Add Class 1 employee NI at 2% above the Upper Earnings Limit (£50,270), and the true combined marginal rate is 62%.

To put this into perspective: the additional rate of income tax for earnings above £125,140 is 45%. So by earning £110,000 you pay a higher marginal rate than someone earning £500,000.

Tax breakdown at key income levels

* Above £125,140 the Personal Allowance is fully tapered away, so the marginal rate drops back to 45% income tax + 2% NI = 47%.

Look at the jump from £100,000 to £105,000. An extra £5,000 of gross income produces an extra £3,000 in income tax (not £2,000 as you’d expect at 40%) plus £100 in NI. Of that £5,000 raise, you keep just £1,900. The effective rate on that slice is 62%.

3. The Childcare Cliff Edge at £100,000

The trap gets worse if you have children. Crossing £100,000 in adjusted net income triggers the loss of Tax-Free Childcare:

Tax-Free Childcare

The government tops up your childcare spending by 20%, up to £2,000 per child per year (£4,000 for a disabled child). If either parent earns over £100,000 adjusted net income, the entire household loses eligibility. For a family with two children, that is £4,000 per year gone instantly.

What about 30 hours free childcare?

A common misconception: the 30 hours free childcare entitlement for 3–4 year olds (and now extended to younger children) is not affected by income. All eligible working parents keep their free hours regardless of earnings. It is only Tax-Free Childcare that has the £100,000 cliff edge.

The Tax-Free Childcare penalty

A parent earning £101,000 with two young children could be over £13,000 worse off per year than one earning £99,999 — even before the extra income tax from the Personal Allowance taper. This cliff edge is one of the most punishing features of the UK tax system for working families.

4. The Solution: Pension Contributions to Escape the Trap

The most effective strategy is to make pension contributions that bring your adjusted net income back below £100,000. Pension contributions are one of the most powerful deductions available under HMRC pension tax relief rules.

Worked example: £115,000 salary

Suppose you earn £115,000 and your employer offers salary sacrifice into a workplace pension. You contribute £15,000 via salary sacrifice, reducing your gross salary to £100,000.

Let’s break down the savings from the sacrifice:

• Income tax saved: £9,000 (£15,000 × 60% effective rate in the taper zone)
• Employee NI saved: £300 (£15,000 × 2%)
• Total tax & NI saved: £9,300
• Pension contribution: £15,000 goes into your pension
• Effective cost of the £15,000 pension contribution: just £5,700 of lost take-home pay

For every £1 of take-home pay you give up, you get £2.63 into your pension. If your employer also saves their 13.8% employer NI on the sacrifice (£2,070), many will pass some or all of that saving into your pension too.

If you have children, bringing your adjusted income below £100,000 also restores Tax-Free Childcare eligibility — worth £2,000 per child per year (£4,000 for two children). Note: the 30 hours free childcare entitlement is not income-dependent.

5. Real-World Scenario: IT Contractor Earning £120,000

You’re an IT contractor on a permanent contract (or inside IR35), earning £120,000 per year. You have two children aged 2 and 4. Your employer offers salary sacrifice.

Scenario A: No salary sacrifice

Scenario B: £20,000 salary sacrifice into pension

The net comparison

By sacrificing £20,000 of gross salary, the contractor loses £7,600 of take-home pay but gains £20,000 in their pension and restores £4,000 in Tax-Free Childcare (2 children) — a total financial benefit of approximately £16,400. The effective cost of putting £20,000 into a pension is just £7,600.

This is likely one of the highest-value financial optimisations available to UK taxpayers in 2026/27. If you earn in this range, this strategy deserves serious attention.

Practical Tools That Can Help

Speaking from experience as someone who runs a limited company outside IR35, managing the £100k threshold is not something you want to do on the back of an envelope. The stakes are too high — one miscalculation and you lose your Personal Allowance, your childcare benefits, or both.

Here are three things that have made a genuine difference in managing tax complexity around this threshold:

• Cloud accounting software — Having real-time visibility of your income against the £100k threshold is essential. Tools like Xero let you track salary, dividends, pension contributions, and expenses throughout the year so you know exactly where you stand before it is too late to act. Your accountant can access the same data, which makes year-end planning significantly easier.
• Private health insurance — If you are earning in this range and running a limited company, business health insurance is a legitimate company expense. Beyond the tax efficiency, the practical benefit is meaningful — NHS waiting times can stretch to months for specialist referrals, and having the option to be seen quickly gives genuine peace of mind when you are the sole earner in your business. Vitality offers a refer-a-friend scheme where we both receive a £100 voucher if you sign up.
• Multi-currency banking — If any of your income arrives in foreign currency (international clients, overseas dividends, or bonuses paid in USD/EUR), the exchange rate and fees matter. Wise offers the real exchange rate with transparent, low fees — and the business account integrates with most accounting software.

These are tools we use or recommend based on personal experience. Individual and company circumstances vary — what works for one person may not be appropriate for another. This is not financial advice. Always check with a qualified accountant before making decisions about your tax position, particularly around the £100k threshold where the consequences of getting it wrong are significant.

Key Takeaways

1. The £100k–£125,140 zone carries a 60–62% effective marginal tax rate due to the Personal Allowance taper — higher than the additional rate band.
2. Crossing £100,000 triggers loss of Tax-Free Childcare (£2,000/child/year). The 30 hours free childcare entitlement is not affected by income.
3. Salary sacrifice pension contributions are the most effective way to reduce adjusted net income below £100,000, restoring the full Personal Allowance and Tax-Free Childcare eligibility.
4. The effective cost of pension contributions in this zone is just 38p per £1 — an extraordinarily efficient way to build retirement savings.
5. Always check your adjusted net income, not just your base salary. Bonuses, benefits in kind, and other taxable income all count towards the £100,000 threshold.

Sources

• HMRC — Income Tax rates and Personal Allowances
• HMRC — Adjusted net income
• GOV.UK — Tax-Free Childcare
• HMRC — Pension tax relief