Completing your Self-Assessment tax return can feel simple at first. You gather your income, add your expenses, check your details and submit everything to HMRC. But small mistakes can quickly lead to stress, penalties, overpaid tax or questions from HMRC.
If you are self-employed, a landlord, a company director, a partner in a business, or someone with extra income outside PAYE, it is important to get your return right. You do not need to make the process complicated, but you do need to be organised.
Working with experienced accountants in Slough can help you avoid common errors, especially if your income comes from more than 1 source or your records are not fully up to date.
HMRC says you must usually tell them by 5 October if you need to complete a tax return for the previous tax year and have not sent one before. The main online filing and payment deadline is 31 January, while paper returns are usually due earlier, by 31 October.
Missing The Registration Deadline
One of the first mistakes is not registering for Self-Assessment on time. Many people think they only need to act when the tax return deadline arrives in January. In reality, if you need to submit a return and have not registered before, you normally need to tell HMRC by 5 October after the end of the tax year.
For example, if you started earning self-employed income during the 2025/26 tax year, which runs from 6 April 2025 to 5 April 2026, you would usually need to register by 5 October 2026.
Leaving this too late can cause problems because you need time to receive your Unique Taxpayer Reference, set up your online account and prepare your figures properly.
Leaving Your Tax Return Until January
The 31 January deadline catches many people out. It is easy to assume you have plenty of time, then suddenly realise you are missing invoices, bank statements, pension details or property income records.
Submitting earlier gives you more control. HMRC confirms you can send your tax return any time after 5 April, and doing it early can help you find out what you owe, budget for payment and set up a payment plan if needed.
You do not have to pay your tax the moment you submit early, as long as you pay by the deadline. The benefit is that you know the figure in advance instead of facing a surprise bill at the end of January.
Forgetting To Include All Income
Your tax return is not just about your main job or main business. You may need to include income from different sources, depending on your situation.
This can include:
- Self-employed income
- Rental income
- Dividends
- Savings interest
- Foreign income
- Capital gains
- Partnership income
- Side income from freelance work
- Income from online selling or platforms
A common mistake is assuming small amounts do not matter. HMRC requires certain people to submit a return, including sole traders earning more than £1,000 before deducting expenses, business partners and people who need to pay Capital Gains Tax.
If you are unsure whether income should be reported, it is safer to check before submitting rather than guessing.
Claiming Expenses That Are Not Allowable
Expenses can reduce your taxable profit, but only if they are allowable. This is where many Self-Assessment mistakes happen.
You should only claim costs that are genuinely linked to your business, rental property or taxable activity. For example, a sole trader may be able to claim business travel, software, phone costs, office supplies, insurance and professional fees. But personal spending should not be included.
If something has both personal and business use, you usually need to claim only the business proportion. For example, if your mobile phone is used partly for work and partly for personal calls, you should not normally claim the full bill unless it is genuinely used only for business.
Not Keeping Proper Records
Good records make your tax return easier, quicker and more accurate. Poor records create guesswork, and guesswork can lead to mistakes.
You should keep invoices, receipts, bank statements, mileage records, payroll information, dividend vouchers, pension contribution records and any details linked to income or expenses.
If you are a landlord, you should also keep records of rent received, repairs, letting agent fees, mortgage interest, insurance, service charges and other property-related costs.
Do not rely only on memory. By the time January arrives, it can be difficult to remember what a payment was for, especially if your bank statement has unclear descriptions.
Mixing Personal And Business Spending
If you are self-employed or run a small business, mixing personal and business spending can make your tax return harder than it needs to be.
A separate business bank account can make things much clearer. Even if you are not legally required to have one as a sole trader, it can help you track income and expenses properly.
When everything is mixed into 1 account, you may miss allowable expenses or accidentally include personal costs. Both can cause problems. One may mean you pay too much tax. The other may increase the risk of HMRC asking questions.
Getting Payments On Account Wrong
Payments on account can surprise people who are new to Self-Assessment. If your tax bill is high enough and you do not pay most of your tax through PAYE, HMRC may ask you to make advance payments towards the next tax year.
This can make your January bill feel much bigger than expected, because you may need to pay:
- The balancing payment for the previous tax year
- The first payment on account for the next tax year
A second payment on account is usually due by 31 July.
If your income has dropped, you may be able to reduce your payments on account. But you need to be careful. If you reduce them too much, HMRC may charge interest on the underpaid amount.
Entering Figures In The Wrong Boxes
Self-Assessment forms can be detailed, especially if you have property income, self-employed income, employment income, dividends or capital gains.
Putting figures in the wrong section can affect your final tax calculation. For example, confusing gross income with net income, entering expenses in the wrong category or missing tax already paid through PAYE can all change the result.
Before you submit, take time to review every section. Check that your figures match your records and that you have not duplicated income or missed deductions.
Not Budgeting For The Tax Bill
A correct tax return is important, but so is being ready to pay the bill.
A common mistake is treating all income as available cash. If you are self-employed, a landlord or receive untaxed income, you should regularly set money aside for tax and National Insurance.
For many people, putting aside a percentage of income each month can reduce stress. The exact amount depends on your profit, other income, tax rate and personal circumstances, but the habit is more important than waiting until January.
If your tax bill is more than expected, do not ignore it. HMRC may allow some taxpayers to set up a payment plan, depending on their circumstances.
Missing The Filing Or Payment Deadline
Late filing can become expensive. HMRC’s late filing penalties include an initial £100 penalty, daily penalties of £10 per day after 3 months up to £900, and further penalties after 6 and 12 months.
Late payment can also lead to interest and further charges. This is why it is usually better to submit your return as early as possible, even if you cannot pay the full amount straight away.
If you are struggling, get advice before the deadline passes. Doing nothing normally makes the situation worse.
Not Checking The Return Before Submitting
Before you press submit, take a final look at the full return. Check your name, address, National Insurance number, Unique Taxpayer Reference, income, expenses, tax already paid and bank details.
Small typing errors can cause unnecessary delays. A missing digit, wrong figure or incorrect box can change your tax calculation.
It is also worth comparing this year’s figures with last year’s. If your income or expenses have changed significantly, make sure there is a clear reason. HMRC may question figures that look unusual, so your records should support what you submit.
Final Thoughts
Self-Assessment does not have to be stressful, but it does need care. Most mistakes happen because people leave things too late, do not keep records, miss income, claim the wrong expenses or misunderstand deadlines.
If you stay organised throughout the year, keep clear records and check your figures properly, you can make the process much smoother. You will also have a better idea of what you owe and when you need to pay it.
If your income is more complex, or you are unsure what applies to you, getting professional support can save time and reduce the risk of costly errors. A well-prepared tax return is not just about meeting HMRC’s rules. It also gives you a clearer view of your finances and helps you plan ahead with more confidence.