Step-by-Step Guide: File Self Assessment Tax Return Online Before 31 Jan 2026

Many might find the self assessment tax return deadline overwhelming. Some would not have yet started compiling the receipts, invoices and bank statements. Others wouldn’t know how to file self assessment tax return.

This guide will convert- once a looming, compliant task to something that is simple and easy to comply – without taking too much of your time. So, whether you are a freelancer, a property owner, or maybe even a company director, this guide is for you.

We will cover the entire process of filing your self assessment tax return online- from registration to submission, all while avoiding common mistakes most taxpayers commit. This guide aims to make your 31st Jan 2026 day, as every other day – probably more efficient and peaceful. Let’s dive in.

Understanding Self Assessment and Who Needs to File

Before we move into the filing process, we must first work on making our basics/foundation strong. Self Assessment is the HMRC’s system to collect income tax from individuals whose income isn’t automatically taxed through PAYE (Pay As You Earn). Simply put, unlike employees whose taxes are deducted from their wages automatically; individuals who are self employed, are a partner, or maybe receive rental income among other scenarios – need to calculate and report their income annually by themselves.

Here’s a brief on who all needs to file self assessment tax return:

Let’s understand with an example of Sarah – a freelance graphic designer.

She started freelancing in June 2024, and by April 2025, her gross income rose to around £45,00.

Since her income exceeded the £1,000 threshold, she must file self assessment tax returns. Additionally, her spouse claims Child Benefit from her of £1,331. Since the HICBC has a £60,000 threshold, and in Sarah’s case, her income is still under the threshold, she only has to pay £1,331 to her spouse.

However, if her income exceeds the threshold in the given future, she then has to pay some, or all the benefit back to HMRC as well. Let’s assume her income rose to £65,000 – which is £5,000 more than the threshold.

The calculation would be £5,000 ÷ £200 = 25% (For every £200 above £60,000, she repays 1% of the Child Benefit).

In the end, her spouse continues to receive the full £1,331 from HMRC, and additionally she pays back £553 to HMRC through self assessment.

Hope you all understood what self assessment is and who all needs to file it. Before wrapping up this section, here are the key deadlines to remember always, if you are filing a self assessment tax return.

• 5 October 2025 – Register for Self Assessment (if you’re filing for the first time)
• 31 January 2026 – Deadline to file online and pay any tax owed (we are going to be discussing this)
• 31 July 2026 – Payment on account deadline (if applicable).

Self Assessment Registration and Login

Step-by-Step Process to File Self Assessment Tax Return Online

Finally, let’s dive into the main event- your actual reason to come to this post – How to file your returns online :

Step 1: Gather all Documents

Collect and compile all necessary documents in one place. This could include- your income records like invoices, sales receipts, P60s, P45s, expense receipts (like business costs, travel expenses, etc), or property and investment details like rental income, capital gains/losses, other expenses etc.

Step 2: Log In to HMRC’s Online Portal

To login to your HMRC’s online portal, you have to visit your government gateway account. You may be asked for photo identifications like passport or driving license to verify your identity

Step 3: Choose Your Tax Year

Precisely choose the tax year- in this case, from 6 April 2024 to 5 April 2025 for Jan 31, 2026 deadline. This can be done by selecting “Self Assessment tax return” from your account’s homepage.

Step 4: Enter Your Income Details

This is where all the bulk information goes. For self employed -they have to enter their gross turnover, and business expenses which they would like to deduct. For others, they have to input earnings from employment, savings, and any additional sources such as property income, bank interest, dividends, etc.

Ex: Sarah entered her £45,000 gross income, then claimed £8,200 in allowable expenses-including £3,500 for equipment, £2,400 for software subscriptions, £1,800 for professional development, and £500 for marketing costs. Her taxable profit in the end was: £36,800.

Step 5: Claim Tax Reliefs and Allowances

In addition to claiming business expenses such as equipment cost, subscription and marketing costs, Sarah can also claim other tax reliefs and allowances. These include – personal allowance which is £12,570, trading allowances, marriage allowance transfer, pension contributions and gift aids.

Step 6: Review Your Calculation

Once HMRC calculates your income tax liability and NI contributions, you then have to carefully review the information – both from your end as well as HMRC’s end. Ensure you don’t miss any eligible deductions.

Step 7: Submit Your Return

Now, you submit your return electronically. HMRC will immediately send an acknowledgement email with your submission reference number. Note this and keep it with you as a proof of filing.

Step 8: Pay Your Tax Bill

Ensure to pay what you owe by 31st Jan. You can use multiple payment methods like bank transfer, credit care, direct debit, telephone banking etc.

Paper vs Online Self Assessment Filing – Which Should You Choose?

Throughout this blog, our focus was on learning to file self assessment tax returns electronically. Let’s understand why.

Paper returns can be an option, but when compared to online self assessment tax return filing, they are slower, more prone to errors, and far more tedious. Here’s a tabular comparison between paper vs online self assessment filing- further strengthening your choice of online method:

There are, however, certain instances, where paper return might be necessary.

• Sarah has to file a paper return if she expects multiple gains from investment bonds and offshore investment funds.
• If she was appointed to be an executor for her late father’s estate, she would then need to file a paper return.
• Furthermore, if she were to move abroad, and become a non-resident taxpayer with UK-source income, her tax situation might require her to file a paper return due to complex residency rules and foreign tax credit relief.

Avoid These Common Mistakes When Filing Online

• Missing the registration deadline: If you are a first-time filer, you must register for self-assessment by 5th October. Failing to do so will result in penalties and fines.

• Missing Income: Ensure you include all your income from employment, self-employment, dividends, and other sources. If you forget to report any, HMRC could issue a penalty.

• Not claiming all allowable expenses: Many self-employed individuals under-claim expenses, resulting in a higher tax liability.

• Incorrect Bank Details: Make sure your bank details are correct, especially when you are due a refund.

• Forgetting about payments on account: If your tax bill exceeds £1,000, you’ll make two advance payments toward next year’s tax. Factor these into your budget.

• Leaving it until the last minute: If you are still surrounded by receipts, statements and scattered invoices, your last-minute effort could go to waste with a single technical issue, forgotten password or missing document. Thus, file early on. You can start submitting your self assessment tax return online from 6th April.

Missed the Deadline? Here’s What You Can Do Next

1. File as soon as you can: Even if it’s late, the sooner you file, the less you’ll pay in fines.
2. Pay what you can: Even if you can only pay a partial amount for now, pay what you can. The longer you delay, the higher the penalties.
3. While HMRC doesn’t accept excuses for late filing, if there are reasonable grounds like you were seriously ill, faced a family emergency, or had technical difficulties – contact HMRC and explain your situation. In some cases, you may be able to request an extension or have your penalties reduced.

How Julian Hobbs & Co. Can Help You File Online with Confidence

We know that every taxpayer’s circumstances differ, and we are here to help with that. Whether you are a sole trader, freelancer or SME, our Hertfordshire accountants can help with deadline management, tax planning and optimisation, and precise filing well ahead of time – saving you time, money and hassle.

Tax legislation changes frequently and with the Autumn Budget 2025 on November 26th- subtle, yet necessary changes are bound to happen. To discuss how we can support your Self Assessment needs, or other accounting and advisory needs, contact Julian Hobbs & Co. today. Let us manage your end-to-end accounting and tax needs, while you focus on growing your business sustainably. If HMRC were ever to enquire about your return or initiates an investigation, we will represent you and provide documentation and explanations on your behalf. This is highly unlikely though, especially if you partner with us from the start.

People Also Ask: