Completing a Self Assessment tax return can feel complicated, particularly when you have several sources of income, business expenses or unfamiliar tax rules to consider. Missing information or entering figures incorrectly may lead to an inaccurate tax bill, additional correspondence from HM Revenue and Customs, or avoidable penalties.

SAS Accountants offers professional Self Assessment tax return services for individuals, sole traders, landlords, company directors and other UK taxpayers. From reviewing your financial records to preparing and submitting your return, the team provides practical support throughout the process.

What Is a Self Assessment Tax Return?

Self Assessment is the system HMRC uses to collect Income Tax from people whose tax is not fully deducted automatically through PAYE.

A tax return normally reports income and gains received during a particular tax year, which runs from 6 April to 5 April. The return may include earnings from self-employment, rental properties, investments, overseas sources and the sale of chargeable assets.

HMRC uses the submitted information to calculate how much tax is due. In some cases, the return may also show that the taxpayer has overpaid and is entitled to request a refund.

Who Needs to Submit a Self Assessment Tax Return?

You may need to submit a return if you are self-employed and your gross trading income exceeds the relevant trading allowance. Other common reasons include receiving rental income, becoming a partner in a business partnership, making taxable capital gains or receiving income that has not already been taxed.

A return may also be required if you have overseas income, significant savings or investment income, or a liability under the High Income Child Benefit Charge. Personal circumstances differ, so it is important to check the current HMRC rules rather than assume that PAYE employment automatically removes the need to file. HMRC provides an online service to help people determine whether a return is required.

If HMRC has issued a notice requiring you to file a tax return, you generally need to submit it unless HMRC formally withdraws that notice.

Registering for Self Assessment

People who are submitting for the first time usually need to register with HMRC. HMRC then issues a Unique Taxpayer Reference, commonly known as a UTR, which is used when managing the return.

For a person who needs to file for the first time, the usual registration date is 5 October following the end of the relevant tax year. Registration should not be left until the final filing deadline because receiving the necessary details can take time.

SAS Accountants can guide new taxpayers through the registration process and help identify which parts of the return apply to their circumstances.

Important Self Assessment Deadlines

For the 2025/26 tax year, which ended on 5 April 2026, the principal online filing and payment deadline is 31 January 2027. HMRC must receive the return and any outstanding balancing payment by that date.

Paper returns generally have an earlier deadline than online submissions. Taxpayers should also consider payments on account, which may create payment dates on both 31 January and 31 July.

Filing early does not normally mean that the tax must be paid immediately. Instead, it provides more time to understand the amount due, plan cash flow and address missing information before the deadline.

What Information Is Needed?

An accurate return depends on complete and well-organised records. The documents required will vary, but they may include:

Self-employment sales and expense records

Employment forms such as a P60 or P45

Details of benefits shown on a P11D

Property income and allowable property expenses

Bank interest and investment statements

Pension income

Dividend certificates

Capital gains calculations

Pension contribution and charitable donation records

Details of overseas income

Student loan information

Taxpayers must keep appropriate supporting records, such as receipts and bank statements, so that the return can be completed correctly. HMRC may impose a penalty if required records are incomplete, inaccurate or unreadable.

SAS Accountants can provide a tailored checklist based on the client’s sources of income, helping to reduce delays and missing entries.

Claiming Allowable Business Expenses

Self-employed taxpayers can usually deduct qualifying business expenses from their trading income before taxable profit is calculated. Depending on the type of business, allowable costs may include office expenses, business travel, professional fees, insurance, advertising and certain costs associated with working from home.

However, an expense is not automatically allowable simply because it was paid from a business bank account. Personal expenditure and the private part of mixed-use costs normally require separate treatment.

SAS Accountants can review expenses, identify those that may qualify and apply the correct tax treatment. This helps ensure that legitimate relief is considered without including unsupported claims.

Self Assessment for Landlords

Landlords may need to report rental income and relevant property expenses through Self Assessment. This can include income from residential property, commercial property, furnished holiday accommodation or jointly owned property.

Property tax calculations can become complicated when a landlord has mortgage interest, repairs, agent fees, insurance, periods without tenants or jointly owned rental income. The tax treatment of an improvement can also differ from the treatment of a repair.

SAS Accountants helps landlords organise their property records, calculate taxable rental profit and complete the appropriate property sections of the return.

Capital Gains and Self Assessment

Selling or transferring assets such as property, shares or business interests can result in Capital Gains Tax. The calculation may involve the original purchase cost, improvement expenditure, selling costs, available losses and relevant exemptions or reliefs.

A capital gain may need to be reported even when the taxpayer has already completed a separate property disposal return. Professional support can help ensure that the transaction is disclosed consistently and that eligible costs and reliefs are considered.

Understanding Payments on Account

Payments on account often cause confusion for taxpayers who are new to Self Assessment. They are advance payments towards the following year’s tax bill and are normally based on the previous year’s liability.

This means a first significant Self Assessment payment may include both the outstanding tax for the completed year and an advance payment for the next year. A second payment may then become due on 31 July.

If income is expected to fall, it may be possible to apply to reduce payments on account. The reduction must be reasonable because interest may be charged when payments are reduced too far.

SAS Accountants can explain each part of the amount due and help clients make informed cash-flow decisions.

Common Self Assessment Mistakes

Errors often arise when taxpayers leave their returns until January or rely on incomplete records. Common problems include omitting a source of income, using figures from the wrong tax year, claiming personal expenses, overlooking taxable benefits and forgetting to declare capital gains.

Other taxpayers incorrectly assume that information already held by HMRC does not need to be included. Although HMRC may receive information from employers, banks and other organisations, the taxpayer remains responsible for submitting a complete and accurate return.

A professional review can identify inconsistencies before the return is filed.

Penalties for Filing Late

A late Self Assessment return can trigger an initial £100 penalty. Further penalties may arise when the delay continues, while unpaid tax can also attract interest and separate late-payment charges. The initial filing penalty can apply even where no tax is due.

Taxpayers who have missed a deadline should take action promptly. Filing the return and addressing the outstanding payment can prevent the position from becoming more serious.

Making Tax Digital for Income Tax

Making Tax Digital for Income Tax began applying from 6 April 2026 to qualifying sole traders and landlords whose combined gross income from self-employment and property exceeded £50,000 in the 2024/25 tax year. Those affected must maintain digital records and use compatible software to meet the applicable reporting requirements.

The rules are being introduced in stages, so more taxpayers may enter the system in future years. SAS Accountants can help businesses and landlords understand whether the requirements apply and prepare their records for digital reporting.

How SAS Accountants Can Help

SAS Accountants provides a complete Self Assessment service tailored to each client’s circumstances. Support may include registering with HMRC, reviewing financial records, calculating taxable income, checking allowable expenses, preparing the return and submitting it online.

The team can also assist with rental accounts, capital gains, payments on account, HMRC correspondence and planning for future liabilities.

Working with an accountant does not remove the taxpayer’s legal responsibility for the return, but it can make the process clearer, more organised and less time-consuming.

Prepare Your Tax Return with Confidence

Self Assessment becomes much easier when records are organised and the work begins early. Whether you are filing for the first time or managing several sources of income, professional support can help you understand your obligations and submit accurate information on time.

Contact SAS Accountants for dependable Self Assessment tax return support and a professional accountancy service with a personal approach.

Call: 0330 133 0278
Website: https://sasaccountant.com/

Frequently Asked Questions

1. Can SAS Accountants submit my tax return on my behalf?

Yes. Once the necessary HMRC authorisation and financial information are available, SAS Accountants can prepare and submit your return on your behalf.

2. Do I need a tax return if I am employed?

Not everyone in PAYE employment needs to file. However, a return may still be required if you have self-employed earnings, rental income, taxable gains, overseas income or another untaxed source of income.

3. When should I give my records to my accountant?

You should provide them as early as possible after the tax year ends. Early preparation gives your accountant more time to check the figures, request missing documents and calculate the amount due.

4. Can I correct a return after submitting it?

A Self Assessment return can normally be amended within 12 months of the filing deadline. The method depends on whether the original return was submitted online or on paper.

5. What happens if I cannot pay my tax bill?

You should contact HMRC as soon as possible. Depending on your circumstances, HMRC may discuss a payment arrangement, although interest or charges may still apply. SAS Accountants can help you understand the balance and organise the information needed before you contact HMRC.