How to File Your Self Assessment Tax Return Online for 2025/26

To file your 2025/26 Self Assessment tax return online, register with HMRC if you have not filed before, sign in to HMRC online services with your Government Gateway or GOV.UK One Login details, then complete and submit the return using your 10-digit Unique Taxpayer Reference (UTR). The deadline for online returns is 11:59pm on 31 January 2027, and any tax you owe must be paid by the same time.

This guide takes you through each step for the 2025/26 tax year (6 April 2025 to 5 April 2026): registering, signing in, what you need, the sections of the return, submitting, paying, keeping records, and the mistakes that most often cause penalties.

What Self Assessment is

Self Assessment is the system HMRC uses to collect Income Tax that is not taken automatically from wages and pensions. You report your income for the tax year after it ends on 5 April, HMRC calculates what you owe from what you report, and you pay by 31 January (GOV.UK).

Who needs to file a Self Assessment tax return

You must send a 2025/26 return if, in that tax year, any of these applied (GOV.UK):

• you were self-employed as a sole trader and earned more than £1,000 before taking off anything you can claim tax relief on
• you were a partner in a business partnership
• you had to pay Capital Gains Tax on something you sold or disposed of
• you had to pay the High Income Child Benefit Charge and do not pay it through PAYE
• you are an off-payroll worker repaying a student or postgraduate loan

You may also need to file if you had untaxed income such as rent, tips and commission, savings interest, dividends or foreign income, and you must file if HMRC asks you to. Our guide to whether you need to complete a tax return covers the grey areas.

Step 1: Register for Self Assessment

You must tell HMRC by 5 October 2026 that you need to file for 2025/26 if you have not sent a tax return before, or you registered before but did not need to send one for 2024/25. You do this through GOV.UK's register for Self Assessment service. If you are starting a business, see how to register as self-employed.

• Your UTR: HMRC usually posts your 10-digit UTR around 15 days after you register, longer if you live overseas (GOV.UK). If you have registered before, it is in your personal tax account, the HMRC app and on earlier HMRC letters. See how to find your UTR.
• Reactivating an old account: if you registered in the past but did not send a return last year, you may need to reactivate your Self Assessment account. The registration service explains how, and filing without reactivating can delay your return.
• Registering after 5 October 2026: HMRC gives you 3 months from the date of its letter or email to file, but the tax is still due by 31 January 2027. If you register late and do not pay in full by then, you may get a failure to notify penalty (GOV.UK).

Step 2: Sign in or create your HMRC sign in details

GOV.UK now describes two ways to sign in to HMRC online services (GOV.UK):

If you already use HMRC online services, keep using the sign in details you have. If you have never used them, choose Create new sign in details when you sign in, even if you have a GOV.UK One Login from another government service; the service then guides you to create Government Gateway or One Login details. When you sign in to file, you may be asked to prove your identity, usually with photo ID such as a passport or driving licence (GOV.UK).

Security access codes can come by text message, voice call or authenticator app. If you enter the wrong details too many times you are locked out for 2 hours, and helpdesk advisers cannot unlock the account (GOV.UK). HMRC's digital assistant can help with registering, filing and activation codes (GOV.UK). Our guide to registering for HMRC online services covers sign in in more detail.

Step 3: Gather what you need

Have these ready before you start, so you can finish the return in one or two sittings:

• your 10-digit UTR and National Insurance number
• your P60 or P45 from any job, and details of benefits from work such as a company car
• records of self-employed income and business expenses: sales invoices, receipts and bank statements
• rent received and letting expenses, if you let property
• savings interest, dividends and any foreign income
• purchase and sale details for anything you sold that may be liable to Capital Gains Tax
• pension contributions and Gift Aid donations
• your bank details, so HMRC can pay any refund

Step 4: Complete the sections of your return

The main return is the SA100. Online, the service asks questions about your income and adds the sections that apply to you, with help text throughout. On paper, these are separate supplementary pages (GOV.UK forms):

You do not have to complete the return in one go: you can save it and come back. If you do not yet know a figure, for example because you are waiting for a valuation, you can use provisional figures, say so on the return, and correct them later (GOV.UK).

HMRC's free online service cannot be used for every return. You need commercial software or paper forms for a partnership, a trust or estate, if you lived abroad as a non-resident, for multiple chargeable event gains, or if you get income from a trust, are a Lloyd's underwriter or a religious minister (GOV.UK).

Step 5: Check the calculation and submit

Before you submit, read through the tax calculation the service produces. Check that every source of income is included, that figures are in the right boxes, and that expenses are only ones you can claim. Then submit. You can file any time from 6 April 2026, and filing early tells you what you owe while there is still time to budget or set up a payment plan (GOV.UK).

After submitting, the same online service lets you view past returns, check how much you owe and print your tax calculation.

Self Assessment deadlines for 2025/26

All deadlines are 11:59pm on the day (GOV.UK deadlines). See when is the next Self Assessment deadline.

Penalties for filing or paying late

• Return 1 day late: an initial £100 penalty.
• 3 months late: £10 a day, up to a maximum of £900.
• 6 months late: a further 5% of the tax due or £300, whichever is greater.
• 12 months late: another 5% or £300, whichever is greater.
• Tax paid late: 5% of the unpaid tax at 30 days, 6 months and 12 months, plus interest.

If you had a reasonable excuse, you can appeal (GOV.UK penalties).

Step 6: Pay your tax bill

By 31 January 2027 you pay the balance of your 2025/26 tax and, if they apply, your first payment on account towards 2026/27. The second payment on account is due by 31 July 2027. Payments on account are each half of last year's bill, and you make them unless your bill was less than £1,000 or more than 80% of your tax was already paid at source, for example through your tax code (GOV.UK). In your first year this can make January's bill much larger than expected; see payments on account explained.

Ways to pay

You can also pay through the HMRC app. You can no longer pay at the Post Office (GOV.UK). Our guide on how to pay your Self Assessment tax has the details.

Paying in instalments

A Budget Payment Plan lets you pay weekly or monthly by Direct Debit towards your next bill, as long as you are up to date with your last one, and you can pause it for up to 6 months. If you cannot pay a bill you already owe, you may be able to set up a Time to Pay arrangement. Your payment reference is your 10-digit UTR followed by the letter K (GOV.UK).

Paying through your tax code

If you already pay tax through PAYE, owe less than £3,000 and file online by 30 December 2026, HMRC will collect the bill through your tax code in 12 monthly instalments, unless you ask it not to on your return (GOV.UK).

Keep your records

You do not send your records with the return, but HMRC can ask to see them and can charge a penalty if they are not accurate, complete and readable. You can keep them on paper, digitally or in bookkeeping software (GOV.UK).

• Self-employed: keep records for at least 5 years after the 31 January deadline, so until at least the end of January 2032 for a 2025/26 return filed on time (GOV.UK).
• Landlords: the same 5 years after the 31 January deadline applies to rental records (GOV.UK).
• Everyone else: at least 22 months after the end of the tax year for an on-time return, or 15 months after you file a late one (GOV.UK).

If you need to change your return

You can amend an online return within 12 months of the filing deadline, so by 31 January 2028 for 2025/26. Wait 3 days (72 hours) after filing, sign in, choose your Self Assessment account, then 'More Self Assessment details', 'At a glance' and 'Tax return options', and pick the year. After that deadline you write to HMRC instead (GOV.UK).

Common mistakes to avoid

• Leaving registration too late. Missing 5 October can lead to a failure to notify penalty if the tax is not paid by 31 January.
• Leaving sign in until late January. Creating sign in details, getting access codes and proving your identity all take time, and a lockout lasts 2 hours.
• Creating a second set of sign in details. GOV.UK says to keep using the details you already have for HMRC.
• Filing without reactivating an old account. Your return may be delayed.
• Forgetting payments on account. First-year filers often owe the full bill plus half as much again in January.
• Leaving out income. Rent, savings interest, dividends, foreign income and gains all count, not just your main job or business.
• Claiming the property or trading allowance and expenses together. You choose one or the other.
• Using the wrong payment reference. A Direct Debit reference is your UTR followed by K; a wrong one can delay the payment or put it against a different bill.

Making Tax Digital for Income Tax from April 2026

Since 6 April 2026, sole traders and landlords whose qualifying income (self-employment and property income before expenses) was over £50,000 in 2024/25 must keep digital records, send quarterly updates through compatible software and submit their return through that software. The threshold falls to £30,000, based on 2025/26 income, from 6 April 2027. You still submit your 2025/26 return in the usual way (GOV.UK).

How GoForma can help

If you would rather not file it yourself, GoForma's ACCA and AAT qualified accountants have filed more than 7,000 tax returns. A one-off Self Assessment return is £198 plus VAT, with HMRC enquiry support included, and our sole trader packages from £22 a month include your Self Assessment. We are rated 4.9 on Google from 72 reviews. See our Self Assessment tax return service, estimate your bill with the self-employed tax calculator, or book a free consultation.