Self assessment: 6 HMRC tax return mistakes that still cost UK high earners thousands every year
Even seasoned taxpayers can be caught out when routines slip and details are missed ⏰
- Even experienced, high-earning taxpayers make the same self assessment mistakes year after year
- Complex income streams in later life increase the risk of errors and overpaying tax
- Some mistakes can trigger penalties, while others quietly inflate tax bills
- Common pitfalls affect pensions, investments, benefits and capital gains
- With the deadline close, small oversights can become expensive problems
With the self assessment deadline less than two weeks away, many higher earners and retirees will be sitting down to complete a task they have done dozens of times before.
Experience helps, but it can also breed complacency. Even people with significant wealth, multiple income streams and professional advice still make costly mistakes on their tax returns every year.
Some errors lead to penalties and interest. Others simply mean paying more tax than necessary. Here are the most common pitfalls, and what to do now to avoid them.
Forgetting about all income sources
One of the biggest issues for higher earners is complexity. Over time, income often becomes fragmented. You may have employment income, private pensions, the State Pension, rental income, dividends, interest, or occasional consultancy work.
A frequent mistake is assuming HMRC already “knows” about everything, particularly pensions or bank interest. While HMRC does receive some information automatically, that does not remove your responsibility to declare taxable income correctly.
State Pension income, for example, is taxable but not taxed at source. It must be included on your return, and it is often overlooked by retirees completing self assessment for the first time.
How to avoid it: Make a checklist of every income source you have received during the tax year and cross-check it against your return before submission.
Missing out on allowable deductions
High earners often focus on reporting income accurately but pay less attention to deductions and reliefs. This can result in a significantly higher tax bill than necessary.
Commonly missed deductions include:
- Pension contributions that qualify for tax relief
- Gift Aid donations
- Certain professional fees or subscriptions
- Allowable expenses related to rental properties
- Capital losses that can be offset against gains
Gift Aid is particularly important for wealthier taxpayers, as higher-rate relief must be actively claimed through self assessment.
How to avoid it: Review bank statements and donation records for the entire tax year. If in doubt, check whether a payment could be tax deductible rather than assuming it is not.
Not using the personal allowance effectively
For couples, especially in retirement, tax efficiency is often about balance. One partner may be paying higher-rate tax while the other has unused personal allowance or lower tax bands.
Failing to split income-generating assets, where legally possible, can lead to unnecessary tax. This is common with savings accounts, investments or rental income held solely in one name.
How to avoid it: Consider whether assets can be shared between spouses or civil partners to make better use of both personal allowances and lower tax bands. This needs to be done properly and ahead of time, but it is worth reviewing annually.
Falling foul of the High Income Child Benefit Charge
Some higher earners still underestimate or forget about the High Income Child Benefit Charge. If you or your partner received Child Benefit and one of you earns over £50,000, a charge may apply, rising to 100% once income exceeds £60,000.
Even if you opted not to receive the payments, you may still need to register for self assessment.
How to avoid it: Check your adjusted net income carefully and ensure the charge is calculated correctly. Pension contributions and Gift Aid can sometimes reduce income below the threshold.
Errors with capital gains
Selling shares, second homes or other assets can trigger Capital Gains Tax, and mistakes here are common. Some people forget to declare gains entirely, while others fail to deduct allowable costs such as purchase fees, improvement costs or annual exemptions.
Another issue is assuming that gains within ISAs or pensions are taxable, when they are not, or conversely assuming everything is sheltered when it is not.
How to avoid it: Keep clear records of asset purchases and sales, and double-check which wrappers provide tax protection.
Leaving it too late
Finally, timing itself is a mistake. Rushing a return days before the deadline increases the chance of errors and removes opportunities to plan. Once the tax year has ended, options to reduce tax are more limited.
How to avoid it: Submit as early as possible, even if you cannot pay immediately. This gives you time to review figures calmly, correct mistakes, and plan for future years more efficiently.