Self Assessment tax returns are used to report income, gains and tax reliefs that HM Revenue and Customs cannot collect or calculate automatically. For sole traders, landlords, investors and people with several sources of income, the return determines the final tax position for the year.

Although HMRC provides an online filing service, completing a return correctly can still be challenging. Missing income, claiming the wrong expenses or misunderstanding payments on account may lead to an incorrect calculation and an unexpected bill.

Professional support can help you prepare the right figures, claim legitimate tax relief and submit the return before the deadline.

Who Needs to Complete a Self Assessment Tax Return?

You may need to file a return if you are self-employed, receive income from property, belong to a business partnership or have untaxed income from investments, commissions, overseas sources or another activity.

A return may also be required when you dispose of assets and have Capital Gains Tax to report, receive income above certain allowances or need to claim particular tax reliefs.

Being a company director does not automatically mean that a return is required. The director’s personal income and wider circumstances must be considered. However, dividends, rental profits, capital gains or other untaxed income may create a filing obligation.

If HMRC sends you a formal notice requiring a return, you should respond even if you believe no tax is due. Anyone uncertain about their position can use HMRC’s Self Assessment checker.

Registering for Self Assessment

People filing for the first time must normally register with HMRC. Once registration is completed, HMRC issues a Unique Taxpayer Reference, commonly known as a UTR.

For the tax year from 6 April 2025 to 5 April 2026, new taxpayers should generally tell HMRC by 5 October 2026 that they need to complete a return.

Registration should not be left until the online filing deadline. Obtaining the necessary reference numbers and activating an account can take time, particularly when information does not match HMRC’s records.

If you previously submitted returns but no longer need to file, you should ask HMRC to close the requirement. Simply deciding not to submit can result in an automatic late-filing penalty.

Upcoming Self Assessment Deadlines

HMRC must receive paper returns for the 2025/26 tax year by 31 October 2026. The deadline for online returns and payment of the tax due is 31 January 2027.

If you want HMRC to consider collecting an eligible bill through your PAYE tax code, the online return must normally be submitted by 30 December 2026. Conditions and limits apply, so collection through a tax code is not guaranteed.

The current dates are confirmed in HMRC’s Self Assessment deadline guidance.

Submitting the return early does not normally bring the January payment deadline forward. It simply tells you how much you need to pay, giving you more time to budget.

What Information Goes on a Tax Return?

A Self Assessment return can include employment earnings, self-employment profits, rental income, dividends, savings interest, pension income, foreign income and taxable capital gains.

The information required depends on your circumstances. A sole trader may need sales records, invoices, receipts and business bank statements. A landlord may require rental statements, property expenses, finance-cost information and details of periods when the property was occupied or unavailable.

Investors may need dividend vouchers, interest certificates and transaction reports. Construction Industry Scheme subcontractors should provide statements showing gross income and tax already deducted by contractors.

The return must present a complete picture. Reporting one business while overlooking rental or investment income can produce an inaccurate tax calculation.

Allowable Expenses for Sole Traders

Self-employed people are normally taxed on their business profits rather than total sales. Allowable business expenses can reduce the taxable profit when they meet the relevant rules.

These may include qualifying office costs, advertising, insurance, professional fees, staff costs, business travel and equipment expenses. Personal expenditure cannot be claimed simply because it was paid from a business account.

Where a cost has both private and business use, only the qualifying business proportion may be deductible. Evidence should be retained to demonstrate how the amount was calculated.

An accountant can review the nature of an expense and determine whether it should be treated as a normal business cost, capital expenditure or a private payment.

Self Assessment for Landlords

Landlords generally need to report rental income and qualifying property expenses. Common costs can include letting-agent fees, landlord insurance, repairs and certain professional charges.

Improvements are treated differently from routine repairs. Extending a property or adding something that was not there before may be capital expenditure rather than an immediate deduction against rental income.

Individual residential landlords must also apply the specific rules for mortgage interest and other finance costs. The amount paid to a lender should not simply be deducted without considering the current restrictions.

Selling a rental property can create a separate Capital Gains Tax responsibility. Where tax is payable on a UK residential property disposal, a 60-day property return may be required before the annual Self Assessment deadline.

Understanding Payments on Account

A first Self Assessment bill can be larger than expected because it may include both the balancing payment for the completed year and the first payment on account towards the following year.

Payments on account are normally based on the previous year’s relevant tax liability. The first instalment is generally due on 31 January and the second on 31 July.

If income is genuinely expected to fall, it may be possible to reduce the payments. However, reducing them too far can lead to interest when the final liability is calculated.

Taxpayers should review the full calculation rather than assuming that every amount shown relates to the year just completed.

Making Tax Digital and Self Assessment

Making Tax Digital for Income Tax became mandatory from 6 April 2026 for qualifying sole traders and landlords whose total qualifying gross income exceeded £50,000 for the 2024/25 tax year.

Those with qualifying income above £30,000 for 2025/26 are due to enter the system from April 2027. The threshold is scheduled to reduce to £20,000 from April 2028, based on qualifying income for 2026/27.

People within Making Tax Digital must use compatible software to maintain digital records and send quarterly updates. They must still complete the required year-end process and pay tax by the usual deadline.

HMRC explains the phased introduction in its Making Tax Digital for Income Tax guidance.

What Happens If Your Tax Return Is Late?

Missing the filing deadline can result in an initial £100 penalty, even where no tax is due or the tax has already been paid.

If the return remains outstanding for more than three months, daily penalties of £10 can apply up to a maximum of £900. Further penalties may arise after six and twelve months. Late payment can create separate penalties and interest.

HMRC provides the full structure in its Self Assessment penalty guidance.

If a return is already overdue, submitting it promptly can prevent additional penalties from accumulating. Where several years are outstanding, each year should be reviewed separately.

Common Self Assessment Mistakes

Frequent problems include omitting income, entering net rather than gross figures, duplicating expenses and claiming private purchases as business costs.

Taxpayers may also forget CIS deductions, use records from the wrong accounting period or fail to include dividends and bank interest. Landlords sometimes deduct full mortgage payments rather than applying the specific finance-cost rules.

Another common mistake is believing that an accountant can retrieve every necessary figure directly from HMRC. HMRC records may contain employment and pension information, but the taxpayer must still provide complete business, property, investment and overseas records.

Why File Your Return Early?

Early filing provides time to investigate missing information and correct bookkeeping before the deadline. It also reveals the tax bill while there is still time to prepare financially.

If a repayment is due, submitting early may allow it to be claimed sooner. A completed return may also be useful when applying for a mortgage or providing evidence of income.

Filing early does not increase the amount owed. It replaces uncertainty with a confirmed calculation and reduces the risk of rushing through important figures in January.

Self Assessment Support from Tax Consultant

Tax Consultant helps individuals and businesses understand their UK tax responsibilities and prepare accurate Self Assessment tax returns.

Support can include reviewing whether a return is required, registering with HMRC, organising income and expense records, calculating the tax position and submitting the completed return. Assistance may also be available for payments on account, amendments, overdue returns and HMRC correspondence.

Visit Tax Consultant to discuss your Self Assessment requirements and receive advice based on your circumstances.

This article provides general information based on UK rules applying in September 2026. Tax treatment depends on individual circumstances, and tailored professional advice should be obtained where necessary.

Frequently Asked Questions
1. Can an accountant submit my Self Assessment return?

Yes. Once authorised as your tax agent, an accountant can prepare and submit the return. You should still review and approve the figures because you remain responsible for the information provided.

2. Do I need to file if I made no profit?

You may still need to submit a return if HMRC has requested one or if your circumstances meet the filing requirements. A business loss can sometimes be reported and used according to the relevant tax rules.

3. What is the online filing deadline for the 2025/26 tax year?

The online return and tax payment are generally due by 31 January 2027. Paper returns must normally reach HMRC by 31 October 2026.

4. Can I file a tax return without paying immediately?

Yes. Filing and payment are separate obligations. You can submit the return before the payment deadline. If you cannot pay on time, contact HMRC promptly to discuss the available options.

5. What records should I give my accountant?

Provide records for every source of income, including employment documents, business accounts, invoices, expenses, rental statements, investment income, CIS deductions and relevant overseas income. You should also provide details of pension contributions, Gift Aid donations and tax already paid.