Self Assessment is HMRC’s system for collecting income tax and National Insurance from people whose tax is not fully collected through PAYE. For most business owners – whether they are sole traders, partners in a business, or directors of limited companies – Self Assessment is an annual obligation. It is also one that many people find more confusing and more stressful than it needs to be.
This article explains what Self Assessment involves, who needs to complete it, what the key dates are, and what the common mistakes are. The goal is to give business owners a clear enough understanding of the process that it stops feeling like a mystery and starts feeling manageable.
Who Needs to File a Self Assessment Tax Return?
Self Assessment applies to a wider range of people than is sometimes assumed. You need to file a return if any of the following apply:
- You are self-employed as a sole trader and your trading income exceeds £1,000 in the tax year
- You are a partner in a business partnership
- You are a director of a limited company – though note that the company’s Corporation Tax is separate from your personal Self Assessment
- Your income from employment, self-employment, or other sources exceeds £100,000 in the year
- You have untaxed income from savings, investments, rental property, or other sources above certain thresholds
- You receive income from abroad
- You or your partner receive Child Benefit and either of you earns more than £60,000 in the year, triggering the High Income Child Benefit Charge
- HMRC has sent you a notice to file a return, even if you do not think you meet the other criteria
If you are unsure whether you need to file, the safest approach is to check rather than assume you do not. Filing a return unnecessarily has no adverse consequences; failing to file when required results in penalties.
The Key Dates to Know
The Self Assessment tax year runs from 6 April to 5 April. For each tax year, the key dates are:
- 5 October – the deadline to register for Self Assessment if it is your first time filing, so that HMRC can send you a Unique Taxpayer Reference (UTR) in time
- 31 October – the deadline for paper returns, which almost nobody uses any more
- 31 January – the deadline for online returns and for payment of any tax owed for the year just ended, plus the first payment on account towards the following year
- 31 July – the deadline for the second payment on account towards the current year
The 31 January deadline is the one that catches most people, partly because it falls immediately after the Christmas period and partly because the payment on account that falls due at the same time can come as a surprise to those experiencing it for the first time.
What Payments on Account Are and Why They Matter
Payments on account are advance payments towards the following year’s tax liability, and they are one of the most commonly misunderstood aspects of Self Assessment.
If your tax bill for the year exceeds £1,000, HMRC requires you to make two payments on account towards the following year – each equal to half of the current year’s liability. The first is due on 31 January alongside the current year’s settlement. The second is due on 31 July.
In practice, this means that in your first year of Self Assessment, you pay your tax bill and then immediately pay another 50% of it on account. That first January can involve settling one and a half times the expected annual liability in a single payment. Businesses that have not planned for this find it a significant cash flow shock.
The solution is to set aside money for tax throughout the year rather than trying to accumulate it in January. A consistent monthly transfer to a dedicated tax reserve account – based on a reasonable estimate of the annual liability – means the money is there when it is needed. An accountant can help estimate the right amount to set aside as the year progresses.
What the Return Itself Covers
A Self Assessment tax return requires disclosure of all taxable income received during the tax year, not just business income. That includes:
- Trading income from self-employment, after deducting allowable business expenses
- Employment income, if you also have a job alongside your business
- Dividend income from company shareholdings, including your own company
- Interest and savings income above the personal savings allowance
- Rental income from property, after allowable expenses
- Capital gains from the disposal of assets, where relevant
The return also captures claims for reliefs and allowances – the trading allowance, pension contributions, gift aid donations, and others – which reduce the taxable income figure and therefore the tax owed.
Getting the income figures right requires good records throughout the year. A business whose bookkeeping is current will find the figures relatively straightforward to pull together. One whose records are incomplete or behind will find the preparation process slow, error-prone, and potentially expensive in accountancy time.
Common Mistakes That Create Problems
The mistakes that cause the most difficulty with Self Assessment tend to be predictable and avoidable.
Leaving it too late is the most common. The January deadline is fixed, and a return prepared under pressure from incomplete records is more likely to contain errors and less likely to optimise the tax position. Starting the process in October or November, with a full set of records available, produces a better outcome.
Forgetting income is another frequent issue. Business owners who receive income from multiple sources – a side activity, rental income, dividends from investments, bank interest – sometimes focus on the main source and omit others. HMRC receives information from banks, employers, and other sources, and discrepancies between what is reported and what is known tend to be identified.
Overclaiming or underclaiming expenses is also common. Business expenses reduce the taxable profit figure, so getting them right matters in both directions: missing legitimate claims means paying more tax than necessary, while claiming costs that do not qualify creates a risk of correction and penalty if HMRC enquires.
Failing to account for payments on account is the mistake that causes the most acute cash flow difficulty. If payments on account were made last year and the current year’s liability is lower, the overpayment should be claimed as a reduction. If no payments on account were made and a large bill arrives in January alongside the next instalment, the financial impact can be significant.
Whether to File Yourself or Use an Accountant
For straightforward cases – a sole trader with a single income source, modest turnover, and simple expenses – filing Self Assessment independently is entirely feasible. HMRC’s online system is reasonably well designed, and the process is manageable once the records are in order.
As the picture becomes more complex – multiple income sources, property, investments, company dividends, capital gains, or simply higher income where the tax position is more material – the case for professional help strengthens. An accountant does not just complete the return; they review the full picture, identify claims that might otherwise be missed, and make sure the figures are presented in the most accurate and tax-efficient way.
They also take the deadline off the business owner’s plate, which has its own value for people who find the process stressful or who consistently leave it until January.
If Self Assessment feels more complicated or more stressful than it should, get in touch with the team at Atria. We manage Self Assessment for clients across a range of circumstances and can make the process considerably more straightforward.
This article reflects Self Assessment rules and thresholds as at June 2026, including the High Income Child Benefit Charge threshold of £60,000. Thresholds and rules are subject to change. This article does not constitute legal or financial advice. For guidance specific to your circumstances, please speak to a qualified professional.
