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Filing a tax return under the self assessment system in the UK can seem overwhelming, particularly for those unfamiliar with the process. Whether you’re self-employed, a landlord, or have income from other sources outside traditional employment, you may be legally required to file a return each year. The process becomes significantly more manageable when approached in stages, beginning with registration, understanding deadlines, and selecting the correct forms for your financial situation.

This guide series aims to simplify the self assessment process, reduce filing errors, and improve financial organisation. Here, we focus on the foundational steps that set the stage for a smooth filing experience.

Who Needs to File a Self Assessment Tax Return

Self assessment is used to collect Income Tax from individuals who do not have it automatically deducted through the Pay As You Earn (PAYE) system. While many assume that self assessment is only required for the self-employed, the criteria extend to several other categories.

You will likely need to complete a self assessment tax return if any of the following apply to your circumstances:

• You are self-employed and earned more than £1,000 in the relevant tax year

• You are a partner in a business partnership

• You earn income from letting property or land in the UK

• You receive untaxed income such as tips, commissions, or freelance payments

• You earn savings or investment income over a certain threshold

• You receive income from outside the UK

• You made capital gains from selling shares, property, or other investments

• Your total income exceeds £100,000

• You receive Child Benefit and your income is over £50,000

• You need to claim tax reliefs such as for pension contributions or charitable donations

If any of the above apply, you must register for self assessment and submit a return each year, even if you have already paid some tax through PAYE or other means.

When to Register for Self Assessment

Registration is the first formal step in the self assessment process. If you are filing a return for the first time, you must register with HMRC so they can assign you a Unique Taxpayer Reference (UTR). This UTR is essential for managing your tax affairs.

Registration should be completed by 5 October following the end of the tax year for which you are filing. For example, if you earned untaxed income during the 2023–24 tax year (which ended on 5 April 2024), you must register by 5 October 2024 to file a return for that year.

The registration process depends on your individual status:

• If you are a sole trader, register as self-employed

• If you are in a partnership, register as a partner, and ensure the partnership is also registered

• If you are neither self-employed nor in a partnership but still need to file (for example, due to foreign income), you can register under the category “not self-employed”

Once registered, you will be issued a UTR number by post. You will also receive an activation code that enables you to access the online tax return service. Registration and receipt of credentials can take 7 to 10 days, so do not delay this step.

Understanding the UK Tax Year

The UK tax year begins on 6 April and ends on 5 April of the following year. Any income you receive within this period must be declared in the tax return corresponding to that specific year.

Understanding the period your return covers is important for accurate income reporting. It also affects which documents and figures you should include when completing your return.

Filing Deadlines for Self Assessment

There are two key deadlines for submitting a self assessment tax return, depending on whether you choose to file by paper or online.

• Paper returns must be submitted by 31 October following the end of the tax year

• Online returns must be submitted by 31 January following the end of the tax year

For example, for the tax year ending 5 April 2024, the paper return must be filed by 31 October 2024, and the online return by 31 January 2025.

The 31 January deadline also applies to paying any tax owed. If you do not pay your tax by this date, HMRC will charge interest and late payment penalties.

There are additional deadlines for making payments on account if your tax liability is more than £1,000. Payments on account are advance payments for the next tax year, due in two installments: one by 31 January and another by 31 July.

Late Filing Penalties

Missing the self assessment deadline comes with automatic penalties. The initial penalty for filing late is £100, even if there is no tax to pay. If the return remains unfiled for longer than three months, daily penalties of £10 apply for up to 90 days.

If your return is six months late, an additional penalty of 5% of the tax due or £300 (whichever is greater) is applied. After 12 months, another 5% penalty or £300 is charged. Interest is also applied to any unpaid tax from the due date until payment is made in full. To avoid these financial penalties, it’s vital to register and file your return well before the deadline.

Accessing the Online Self Assessment System

Once you have received your UTR and activation code, you can create an account on the HMRC website using the Government Gateway system. After your account is set up, you will be able to:

• Complete your tax return online

• Save your progress and return to it later

• Calculate how much tax you owe

• Submit your return electronically

• Set up or amend payment plans

The system will also suggest which supplementary pages you may need based on your income sources. However, it is still important to understand the different forms to ensure accuracy.

The Main Self Assessment Form: SA100

The SA100 is the primary form used for all individual taxpayers who are filing under self assessment. It includes sections for:

• Personal details such as name, address, and national insurance number

• Income from employment and pensions

• Interest and dividends from UK savings and investments

• Income from state benefits

• Student loan repayments

• Gift Aid donations

• Pension contributions

• Other income not covered by supplementary pages

This form must be completed in all cases, regardless of your income sources. It acts as the base form, with other forms attached as necessary.

Supplementary Forms Based on Income

In addition to the SA100, you may need to complete supplementary pages if your income extends beyond employment or pensions. Each form covers a specific type of income or financial arrangement.

SA102 – Employment Income

This form is used to report income from employment. It includes sections for salary, tax deducted, company benefits, and other payments. You will need information from your employer such as your P60 and P11D.

SA103S or SA103F – Self-Employment

If you run your own business, the version of the self-employment form you use depends on your turnover.

• Use SA103S if your annual turnover is below the VAT registration threshold

• Use SA103F if your turnover exceeds the threshold

These forms allow you to report business income, allowable expenses, capital allowances, and net profit. Maintaining good accounting records throughout the year simplifies this process.

SA104S or SA104F – Partnership Income

If you are a member of a business partnership, you must complete the appropriate version of the partnership form.

• SA104S is the short version for simple partnership arrangements

• SA104F is the full version for more complex income or deductions

Details from the partnership’s own return (SA800) are required to complete this form accurately.

SA105 – Property Income

Individuals receiving income from property or land, such as residential lettings or furnished holiday lets, need to fill in the SA105. It requires you to report rental income, property expenses, and mortgage interest.

SA106 – Foreign Income

If you receive income from outside the UK, whether from employment, interest, dividends, or pensions, it must be declared using the SA106. The form also allows for foreign tax credit relief where applicable.

SA107 – Trusts and Settlements

Income derived from trusts, settlements, or estates should be reported using the SA107. This includes discretionary and interest-in-possession trusts, as well as income from the estate of a deceased person.

SA108 – Capital Gains

The SA108 is used to report profits from the sale or disposal of capital assets, including property, shares, and valuable personal items. You’ll need to include acquisition and disposal values, associated costs, and reliefs claimed.

SA109 – Non-Residence

If you are not resident in the UK for tax purposes but still receive UK income or capital gains, the SA109 should be completed. It includes questions about your residency status and eligibility for split-year treatment.

Choosing the Correct Forms

Selecting the correct forms for your tax return depends on your personal and financial situation. You should consider:

• Whether you have employment income, self-employment income, or both

• If you are part of a partnership or own rental property

• Whether you earned money from abroad or sold any assets

• If you need to claim reliefs or report specific deductions

The online system typically helps guide users through this selection. However, individuals filing by paper must ensure all relevant supplementary pages are completed manually. Errors in form selection can delay processing and may result in penalties or underpayment of tax.

Downloading and Completing Paper Forms

If you choose to file by paper, all forms can be downloaded from HMRC’s website. Each form includes notes to guide you through each section. The paper submission must reach HMRC by the 31 October deadline.

Accuracy is vital when completing paper forms, as they are not automatically validated like online submissions. Always double-check figures, especially when copying from accounting records, bank statements, or employment summaries.

Importance of Preparation

Many individuals leave their tax return to the last minute, leading to missing paperwork, errors in reporting, and ultimately stress. Gathering the necessary data in advance can prevent these issues. A well-organised taxpayer will not only avoid fines and delays but also increase the likelihood of claiming all allowable expenses and reliefs.

Having all supporting evidence at hand before you begin the self assessment return will allow for a much quicker and more efficient process. It also allows for better financial oversight, helping you track profitability and areas where you can cut costs.

Required Personal Information

Before delving into income and expenses, make sure the following personal information is readily available:

• National Insurance number

• Unique Taxpayer Reference (UTR)

• Government Gateway user ID and password

• Name, date of birth, address, and contact details

• Bank account details for tax refund or payments

If you operate a business or are in a partnership, also keep ready:

• Business or partnership name

• Business address

• Company registration number (if applicable)

These basic identifiers are required on every tax return and should match records held by HMRC to avoid rejections or delays.

Income Documentation

Accurately reporting all sources of income is a legal requirement under the self assessment system. Omissions or misreporting can lead to investigations and penalties. You should collect all evidence of earnings throughout the tax year, regardless of how minor the amount may seem.

Employment Income

If you have income from one or more jobs, the following documents are essential:

• P60: This shows your total pay and tax paid for the year

• P45: If you left a job during the year

• P11D: If your employer provided you with benefits or reimbursed expenses

• Payslips: Useful for cross-checking figures if other documents are unavailable

Make sure you include any job-related benefits such as company cars, health insurance, or expense allowances, as they are considered taxable income.

Self-Employment Income

For those who run a business as a sole trader, record-keeping must cover all earnings and expenditure. You should have:

• Sales invoices or receipts

• Records of payments received

• Accounting software reports (if applicable)

• Bank statements for business accounts

• Cashbook or ledger summaries

All income must be declared, even if received in cash or from short-term projects. Properly separated business and personal accounts make it much easier to manage self-employment income and avoid confusion.

Rental and Property Income

If you receive income from property rental or leasing, you must provide detailed figures:

• Rental income from tenants

• Letting agent statements

• Mortgage interest statements

• Invoices for property maintenance or repair work

• Insurance costs for rental properties

• Council tax and utilities (if paid by the landlord)

These records will support the SA105 form and help calculate the taxable income from property after allowable deductions.

Dividend and Savings Income

Many individuals receive income from investments, including dividends, savings interest, or trust income. For accurate reporting, gather:

• Dividend vouchers from UK or foreign companies

• Statements from investment platforms

• Bank interest statements

• Certificates for tax deducted at source

Even if the interest or dividends are small, they must be included. Different rates apply depending on the type of income and your total income band.

Foreign Income

Income originating from outside the UK must also be declared. This includes:

• Foreign employment earnings

• Interest or dividends from overseas accounts

• Rental income from properties located abroad

• Overseas pensions or annuities

• Any taxes paid to foreign governments

You may be eligible for foreign tax credit relief, which prevents double taxation. Currency conversion should be calculated accurately, using exchange rates applicable on the date of transaction or the average rate for the year.

Capital Gains

If you disposed of an asset during the year and made a profit, you must declare the gain. Documents should include:

• Date of acquisition and sale

• Purchase and sale prices

• Incidental costs (solicitor fees, agent commissions)

• Records of improvements made to property

• Calculations of capital gain or loss

• Evidence of capital losses carried forward

This applies to sales of property, shares, cryptocurrency, business assets, and other chargeable items.

Pension and State Benefits

Income from pensions and certain state benefits are taxable and must be declared. Gather:

• Private pension income statements

• Statements from HMRC for state pension

• Details of Jobseeker’s Allowance, Employment Support Allowance, or Universal Credit (if taxable)

• Lump-sum pension payments

Tax-free lump sums should be clearly separated from taxable pension income.

Other Income Sources

Additional forms of income that should be included on your return may involve:

• Tips and commissions

• Freelance work paid outside your main job

• Royalties

• Grants or bursaries (depending on how used)

• Compensation payments

Where relevant, retain contracts, award letters, or invoices to support these amounts.

Claiming Allowable Expenses

One of the most beneficial aspects of self assessment is the ability to reduce taxable income by claiming allowable business expenses. These are costs incurred solely for the purpose of generating business income. Proper documentation must be maintained for each expense claimed.

Travel and Vehicle Costs

If you use a vehicle for business, you can claim for:

• Fuel costs

• Repairs and servicing

• Vehicle insurance

• Road tax and MOT

• Parking fees (not including fines)

• Business-related public transport fares

You can either use actual vehicle expenses or mileage allowance rates, depending on your circumstances. Accurate mileage logs or receipts are essential.

Office and Premises Costs

These expenses may include:

• Rent for business premises

• Home office usage (a proportion of household bills)

• Business rates

• Utility bills for office space

• Phone and internet bills (business use portion)

If using your home for work, you must calculate the business-use percentage based on the number of rooms and hours used.

Staff and Subcontractor Payments

Wages, salaries, pensions, and payments to freelancers or subcontractors can all be claimed if they are for business purposes. You will need:

• Payslips or payroll summaries

• Contracts or agreements with subcontractors

• Invoices or payment confirmations

All payments must be properly recorded and compliant with employment law and reporting requirements.

Equipment, Tools, and Supplies

Purchases necessary to run the business may include:

• Computers and office equipment

• Tools and machinery

• Stationery and printing

• Software subscriptions

• Inventory or stock

Some items may qualify for capital allowances if they are large or used over several years.

Marketing and Advertising

You can claim for:

• Website development and hosting

• Print or online advertising

• Social media campaigns

• Branding and design services

• Promotional materials

Receipts and invoices should clearly show that the expenses were incurred in the course of business.

Financial and Legal Services

Costs in this category might include:

• Accountant or bookkeeper fees

• Solicitor or legal advisor charges

• Bank charges on business accounts

• Interest on business loans or credit facilities

You cannot claim for penalties, fines, or non-business-related legal expenses.

Clothing and Uniforms

Claimable clothing expenses are limited to:

• Protective clothing necessary for your work

• Uniforms with logos

• Costumes for performance-based roles

Standard business clothing or personal fashion purchases are not deductible.

Training and Development

You may be able to claim for training courses, provided they are relevant to your current trade or improve your existing skills. Initial qualification costs are not deductible, but refresher or update courses are.

Keeping Records and Evidence

It is not enough to simply declare figures on your return. HMRC may request evidence, especially if your return is selected for a compliance check. Good record-keeping includes:

• Storing invoices, receipts, and bank statements

• Maintaining logs or spreadsheets for mileage, expenses, and income

• Keeping digital or paper records for at least five years after the 31 January submission deadline

Organised records not only ensure you are compliant but can also help if you ever wish to apply for a loan, mortgage, or grant where proof of income is required.

Digital vs Paper Records

You can choose to keep paper copies of your financial documents or use digital tools. Many businesses now use accounting software, cloud storage, or spreadsheet templates to track income and expenses throughout the year.

Whichever method you choose, consistency is important. Digital records must be clearly organised and backed up, while paper records should be kept in labelled folders with chronological ordering.

HMRC supports the shift to digital tax reporting, and good habits now will prepare you for future requirements under Making Tax Digital.

Common Mistakes in Income and Expense Reporting

When preparing your documents, avoid these common pitfalls:

• Forgetting to declare small or irregular income

• Claiming personal expenses as business deductions

• Misclassifying capital expenses as revenue expenses

• Using estimated figures instead of actual values

• Mixing personal and business finances

Attention to detail now can prevent headaches during filing or even years later if HMRC investigates your return.

Understanding the SA100: The Core Return Form

The SA100 form is the main self assessment tax return form. It is required by anyone who receives taxable income not taxed at source or who must declare other financial activity. The form is broken into multiple sections, with supplementary pages added according to the individual’s circumstances.

The core form covers:

• Personal details

• Income from employment

• Benefits and pensions

• Interest and dividends

• Gift Aid

• Student loan repayments

• Tax calculation overview

Before you begin, ensure you are using the correct version for the tax year in question. If you are filing online, the digital system will guide you through the appropriate sections based on your responses.

Logging Into the Online Tax Return Portal

To complete your return online, visit the official HMRC self assessment portal and log in using your Government Gateway credentials. If this is your first time filing online, you will need to activate your account using the details provided when you registered.

Once logged in, select the relevant tax year and begin the return. The portal allows you to save your progress and return at any time. You should plan for several sessions to complete everything properly, especially if your financial situation is complex.

Filling Out Personal Information

Start by verifying your personal information. This includes:

• Name and date of birth

• Address and contact details

• National Insurance number

• Marital status

• Residency status

• Student loan information, if applicable

Make sure your contact details are up to date. These details are essential for correspondence and for the system to assess whether additional forms or sections are required.

Declaring Employment Income

If you are employed, report your earnings using the information provided on your P60 or final payslip for the tax year. You should also include details from your P45 if you left a job during the year, and from your P11D if you received benefits from your employer.

The form will ask for:

• Gross income before tax

• Tax deducted by your employer

• Benefits and expenses

• Employment expenses you paid yourself

Use the SA102 supplementary page for additional employment income if you held more than one job or earned freelance income under PAYE.

Declaring Self-Employment Income

If you are self-employed, your income and expenses will be reported in either the short (SA103S) or full (SA103F) self-employment section. The short version is designed for those with a turnover under the VAT threshold.

The key sections are:

• Business name and address

• Accounting period dates

• Income received

• Allowable business expenses

• Capital allowances

• Adjustments and losses

Use your accounting records to enter exact figures. If your accounts are based on accrual accounting (rather than cash basis), note this where applicable. If your business made a loss, you may be able to carry it forward or offset it against other income.

Entering Property Income

If you earn income from renting out property, fill in the SA105 section. Enter the total rental income for the year and list allowable expenses such as:

• Mortgage interest

• Repairs and maintenance

• Agent fees

• Utilities paid by the landlord

• Insurance and ground rent

The form will calculate your net income from property after deductions. Note whether the property is jointly owned, as you may only report your share of the income and expenses.

Entering Savings and Investment Income

Income from savings and investments goes into its own section. You must include:

• Interest from UK banks and building societies

• Dividends from UK companies

• Interest from other investments

• Foreign interest or dividends (reported separately)

Most banks and investment platforms issue year-end tax summaries that detail the income you received and tax already deducted. Enter gross amounts before tax deduction unless the form requests net figures.

Declaring Foreign Income

Any foreign income must be entered using the SA106 supplementary page. This includes employment income, pensions, property income, and interest or dividends received from overseas.

You may be able to claim Foreign Tax Credit Relief for tax already paid abroad. To do this correctly, you will need to:

• State the type of income

• Name the country of origin

• Provide the gross income figure

• Declare tax paid to the foreign government

The form will calculate how much tax relief applies. Keep all foreign currency conversion calculations documented.

Declaring Capital Gains

The SA108 section is used to report gains from selling assets such as:

• Property not used as your primary residence

• Shares or securities

• Personal possessions over a certain value

• Business assets

You will be asked to provide:

• Date of acquisition and disposal

• Sale proceeds

• Allowable costs

• Gain or loss made

If you made multiple disposals during the year, keep a separate record of each transaction. Declare any losses that can offset gains or be carried forward.

Declaring Other Income

Other types of income that may need to be included are:

• Trust or settlement income

• Income from partnerships

• Royalties and intellectual property income

• Commission or tips

• Lump sums and windfalls (if taxable)

Use supplementary pages such as SA107 or SA104 as needed. Ensure you specify the nature of the income and the amounts received.

Claiming Reliefs and Allowances

The return includes sections for reliefs and adjustments. These may include:

• Pension contributions

• Gift Aid donations

• Blind Person’s Allowance

• Marriage Allowance transfer

• Qualifying loan interest payments

Enter the full details as required and ensure you retain documents to support the claims. Reliefs reduce your taxable income or allow tax rebates.

Completing the Tax Calculation Summary

At the end of the return, the system will automatically generate a summary of the tax due, based on the income, allowances, and deductions you have entered.

The calculation will include:

• Total income tax owed

• National Insurance contributions

• Payments on account for the next year (if applicable)

• Balancing payment due

• Student loan repayments

• Total tax payable

If you are filing by paper, you may need to complete the calculation yourself using HMRC’s guidelines.

Reviewing Your Return Before Submission

Before you submit your return, take time to review every section. Common errors to check for include:

• Incorrect figures or typos

• Missing supplementary forms

• Misreported income

• Over-claimed expenses

• Missing National Insurance contributions

• Incorrect bank account details

Correct any mistakes and ensure all totals match your documentation. Once submitted, you can amend your return later, but it’s best to minimise the need for changes.

Submitting Your Return

When everything is reviewed, submit the return online through the self assessment portal. You will receive a confirmation and a unique submission receipt.

If you are submitting by paper, the return must be posted to HMRC by the earlier 31 October deadline. Ensure that postage and delivery times are accounted for.

Making a Payment

Tax due must be paid by 31 January following the end of the tax year. There are several options for payment:

• Online banking

• Debit or corporate credit card

• Direct Debit

• Bank transfer

• Cheque (if sending by post)

You must use your 11-digit payment reference, which is your UTR followed by the letter K. Ensure payments are made in time to avoid interest and late payment penalties.

Payments on Account

If your previous year’s tax bill was over a certain threshold, you may be required to make payments on account toward next year’s tax bill. These are advance payments due in two instalments:

• First payment: 31 January

• Second payment: 31 July

Each is typically 50 percent of the previous year’s tax bill. If you expect your income to be significantly lower next year, you can apply to reduce your payments on account.

Setting Up a Payment Plan

If you are unable to pay the full amount by the deadline, you may be eligible for a Time to Pay arrangement. This allows you to spread the cost over monthly installments.

To apply, you must:

• File your return

• Owe less than a certain threshold

• Have no other outstanding HMRC debts

Apply as early as possible to avoid accruing penalties. Interest may still apply, but spreading payments can ease financial pressure.

Receiving a Refund

If your return shows that you have overpaid tax, HMRC will issue a refund. You can choose to:

• Have the refund paid directly to your bank account

• Leave the amount on your account to offset future tax bills

Refunds usually arrive within a few weeks of submission, but it may take longer during peak filing season.

Amending Your Return

After submission, you can make changes to your return within 12 months of the deadline. Log into the online portal and select the option to amend your return.

You can correct:

• Figures or income entries

• Expense claims

• Personal details

Once updated, resubmit the return. You will receive a revised tax calculation and new payment or refund details.

Conclusion

Completing a self assessment tax return may seem daunting at first, but with the right preparation and step-by-step guidance, it becomes a manageable and even empowering process. Whether you’re a seasoned self-employed professional, a first-time freelancer, a landlord, or someone with a range of income sources, understanding your responsibilities and gathering your information early is key to avoiding unnecessary stress.

From the moment you determine whether you need to register, right through to submitting your return and making payment, every stage is an opportunity to take control of your finances. Using the correct forms, knowing your deadlines, and having the right documents at hand ensures accuracy and protects you from penalties or HMRC inquiries.

One of the most valuable habits you can build is maintaining organized records year-round. Logging income, keeping receipts, and tracking deductible expenses as they occur makes year-end filing smoother and more accurate. The more precise your reporting, the better positioned you are to optimize your tax position and potentially reduce your tax bill.

If you miss a deadline or make a mistake, it’s important not to panic. HMRC provides channels to amend your return, and in many cases, arrangements can be made to spread payments if needed. The system is designed to be accessible, and with care, attention to detail, and early action, you can navigate it confidently.

Tax may be unavoidable, but confusion and anxiety around it are not. Take advantage of the tools and resources available to stay organized, understand your obligations, and meet them without last-minute pressure. By following a checklist-based approach and maintaining clarity throughout the process, self assessment can become just another part of running your business or managing your finances effectively on your terms, and on time.