Need to, but have no idea how to file your self-assessment tax return? Today, filing the declaration is easy and simple. Today, you will learn exactly the step-by-step process to file the declaration with ease.
For most workers in the UK, income tax is handled simply and efficiently by the employer through the Pay As You Earn (PAYE) system. However, complexity arises when you generate profits from alternative sources, requiring a personal disclosure of amounts to the Revenue.
Therefore, knowing precisely how to file your self-assessment tax return ensures that you comply with your legal duties and avoid significant financial penalties, which can accumulate quickly.
This complete guide will provide a step-by-step process to master your tax responsibilities. We will detail the exact criteria that define who must declare, review the crucial deadlines you must meet. And you will know the precise details on how to file your Self-Assessment declaration with confidence and accuracy.
Understanding the Self-Assessment System and Declaration Obligations
Self-Assessment is the UK’s primary mechanism for collecting Income Tax when profits are not automatically deducted at the time of payment.
This structure exists because the standard PAYE system cannot account for all types of income sources a person may have.
As a rule, the system operates based on the UK tax year, which runs from April 6 to April 5 of the following year. Individuals and businesses that receive other income must submit a declaration after the end of this cycle.
Who Must Submit a Self-Assessment Declaration? (How to file your self-assessment tax return)
The obligation to complete a Self-Assessment Declaration falls on anyone who received income that was not fully taxed during the tax year.
Initially, HMRC specifically requires submission in several situations.
A main group is the sole trader, meaning anyone who worked as self-employed and earned more than £1,000 before any deductible expenses were considered.
In addition, partners in a partnership must also submit a declaration. Those who made a profit from the sale or transfer of an asset, thus incurring Capital Gains Tax liability, are obliged to declare.
A third specific situation involves the High Income Child Benefit Charge, which affects high-income taxpayers or their spouses who received Child Benefit.
Furthermore, the obligation extends to various forms of untaxed income.
Moreover, anyone who receives income from abroad must also inform the Revenue about these amounts. Additionally.
Although not strictly mandatory, other taxpayers choose to submit a declaration to claim tax refunds. Establishing proof of self-employment status for benefits or paying voluntary National Insurance contributions.
Deadlines you should be aware of for filing the declaration
Compliance with deadlines is arguably the most crucial aspect of the Self-Assessment process, as late filing penalties are rigorously applied.
However, before you can declare, you must first register.
Anyone who has never submitted a Self-Assessment Declaration before must register with HMRC by October 5 after the end of the tax year.
The authority explains that the taxpayer must confirm their need to complete a declaration and, upon registration, will receive a crucial ten-digit code known as a Unique Taxpayer Reference (UTR).
The submission schedule depends on the chosen method.
As a rule, for taxpayers who prefer to submit their documents on paper, the final deadline is October 31, 2025.
Most, however, declare online, and the deadline for digital submissions is significantly later, set for 11:59 PM on January 31, 2026.
A separate and earlier deadline exists for those who wish to arrange tax payment via a discount code applied directly to their payroll. Known as a tax code. These online declarations must be submitted by December 30, 2025. Regardless of the submission method, the deadline for paying the tax due is January 31, 2026.
Table of deadlines and late payment penalties
| Action/Criterion | Deadline (Tax Year 2024/2025) | Initial Late Penalty | Additional Late Payment Penalties |
| Registration (New Declarants) | October 5, 2025 | N/A | N/A |
| Paper Submission | October 31, 2025 | £100 | N/A |
| Online Submission | January 31, 2026 | £100 | N/A |
| Online Submission (Payment via Tax Code) | December 30, 2025 | £100 | N/A |
| Tax Payment Deadline | January 31, 2026 | 5% of unpaid tax after 30 days | 5% after 6 months; 5% after 12 months (plus interest) |
| Daily Submission Penalty | After 3 months late | £10 per day (up to £900) | N/A |
Complete Guide on How to File Your Self-Assessment Declaration
The first thing you should do is check if you need to declare, using the established criteria and HMRC’s online verification tool to confirm your obligation to submit the Self-Assessment Declaration.
If it’s your first time, registration is the next immediate step. This must be completed by October 5 through the HMRC website to obtain your UTR and establish access to your Government Gateway account.
Even before you start filling in the data, you must gather all the necessary documentation and information. Gather the following documents:
- Your UTR (the ten-digit number);
- National Insurance Number.
- Proof of the income you earned, including self-employment profits, dividends, and interest.
You must also have meticulous records of all deductible expenses, along with receipts. And evidence of contributions made to pensions or any eligible donations to charities.
Finally, gather forms such as P60 or P45—these documents show any income that has already been taxed under the PAYE system.
How to fill out the forms? (How to file your self-assessment tax return)
With the necessary documents gathered, you must choose your submission format. The online method is more common.
Firstly, the SA100 is the main form where you inform the Revenue about essential details.
Here, you report both taxed and untaxed income, such as interest and dividends. You also declare pension contributions and charitable donations.
Information about benefits received, such as state pensions or disability benefits, must be included.
Finally, you must detail any relevant information regarding the High Income Child Benefit Charge and the Marriage Allowance, if applicable to your situation.
The next phase requires the addition of supplementary pages if your income sources include categories beyond standard employment and investments.
Taxpayers who declare self-employment income must complete form SA103. While those who report rental property income use form SA105. If you incurred Capital Gains Tax, you will need page SA108.
For sole traders using the SA103, it is necessary to detail gross revenue and all deductible expenses, which are vital for calculating your final tax bill.
Submission and Payment of Your Tax Bill
Once the forms are complete, you can submit your declaration from April 6, immediately after the end of the tax year.
HMRC emphasizes that submitting your declaration early offers advantages. Once you know exactly how much you owe, you can plan payment over several months and can even arrange payments in installments, if necessary.
After submission, HMRC calculates the exact tax due based on current rates.
Full payment is due by January 31.
As mentioned, there is a possibility of payments on account for those who need to split their liability, with the second estimated payment due on July 31.
After the entire process, you must keep all receipts and records for at least five years, as HMRC may request them for verification.
In addition, if you used provisional numbers or subsequently discovered errors, you have the option to correct the declaration up to 12 months after the final deadline.
By following this guide, you will be able to invest in stocks and other assets that will give you current income, such as sustainable investments with low risk, without worrying.
Conclusion (How to file your self-assessment tax return)
In the UK, Self-Assessment is a tax duty for anyone who receives income outside the PAYE system.
Successfully navigating this process, from complying with the October 5 registration deadline to the January 31 payment date. Thus reducing the risk of heavy penalties, significantly facilitates financial management;
Just follow the step-by-step and submit all documents. This way, you will comply with your obligations without any complications.