How to Amend a Self Assessment Return Correctly

A return submitted in a rush can easily contain a missed expense, the wrong income figure or an allowance you only remembered after pressing submit. If you are searching for how to amend self assessment return details, the good news is that HMRC usually gives you a clear route to put things right. The key is acting promptly, keeping evidence for the change and understanding whether it will leave you with more tax to pay or money to reclaim.

For self-employed people, landlords, CIS subcontractors and company directors, an amendment is not unusual. You may receive a late CIS deduction statement, find a mileage record, spot duplicated income or receive corrected figures from a bank or agent. Correcting the return is normally far better than hoping the mistake will go unnoticed.

When can you amend a Self Assessment return?

In most cases, you can amend a Self Assessment tax return up to 12 months after the 31 January online filing deadline for that tax year.

For example, the 2024/25 tax return is due online by 31 January 2026. You can normally amend it until 31 January 2027. This window applies whether you submitted early or close to the deadline.

Do not confuse the amendment deadline with the tax payment deadline. An amendment that increases your taxable profit could create extra tax, National Insurance or student loan repayments. Interest may then run from the original payment due date, which is usually 31 January following the end of the tax year. Paying the extra amount quickly limits the cost.

If your amendment reduces the bill, HMRC may issue or adjust a repayment. That is particularly relevant for CIS workers who discover deductions that were not included on the original return.

If the amendment window has passed

You may still have options, but they are more limited. Depending on the circumstances, you might be able to make a claim for overpayment relief, generally within four years of the end of the tax year concerned. The rules are detailed and claims can be refused where another route was available or HMRC has already dealt with the point.

If HMRC has opened an enquiry into the return, seek advice before making changes. You should still be open and cooperative, but the correct process may differ from a straightforward online amendment.

How to amend a Self Assessment return online

If you filed your return through HMRC's online service, the practical process is usually straightforward. Log in to your Government Gateway account, select the relevant tax year and choose the option to amend the return. You can then revisit the sections that need changing, such as self-employment income, property income, employment pay, pension contributions, CIS deductions or allowable expenses.

Make the correction using records rather than estimates where possible. A taxi driver, for instance, might have found a missed repair invoice or updated mileage evidence. A landlord may have received a final letting agent statement showing fees or rental income were different from the figures first used. A construction subcontractor may have a late deduction statement showing tax already withheld.

Once you have updated the figures, work through the return again and check the revised calculation before submitting. Save or print a copy of the amended return and the new tax calculation for your records. It is sensible to make a short note explaining what changed and why, especially where the change is significant.

HMRC will normally confirm that the amendment has been received. Keep an eye on your online tax account afterwards to see whether the amount due, payment on account or repayment position has changed.

Amending a paper return

If you originally filed a paper return, you cannot simply alter your own copy and keep it on file. You will usually need to send HMRC the corrected pages, clearly marked as an amendment, or write with the full details of the correction. Include your Unique Taxpayer Reference, the tax year, the figures originally declared, the revised figures and the reason for the change.

Paper amendments take longer to process than online changes, so leave enough time before the amendment deadline. Send copies rather than original receipts, invoices or deduction statements unless HMRC specifically asks for originals. Keep proof of posting and retain a full copy of what you send.

Check these areas before you submit again

An amendment is a useful chance to check whether one error points to another. It is easy to focus on a missing expense while overlooking an incorrect turnover figure or a deduction entered twice.

For sole traders and subcontractors, review sales or takings, CIS tax deducted, materials, vehicle costs, tools, insurance, phone use, accountancy fees and use-of-home costs. Expenses must be allowable and connected to the business. Personal spending cannot be claimed simply because it was paid from the business account.

Landlords should check rent received, agent fees, repairs, insurance, finance cost relief and the split between revenue repairs and capital improvements. Replacing a worn-out item may be treated differently from improving a property, so the description on an invoice matters.

Company directors should also check PAYE income reported through payroll, dividends supported by company paperwork, pension contributions and any benefits received. A personal tax return needs to agree with the company records, but it should not duplicate income already included elsewhere.

Do not create a new problem while fixing an old one

The temptation is to claim every cost found after filing. That can backfire. HMRC can ask for evidence, and an unsupported expense can lead to more tax, interest and potentially penalties. If a figure is uncertain, resolve it before amending rather than entering a rough number and hoping it is close enough.

This matters particularly where business and personal costs are mixed. Mobile phones, home working, vehicles and travel can all involve a business-use calculation. The right treatment depends on the facts, not on what produces the biggest tax saving.

What happens to payments, refunds and penalties?

After an amendment, HMRC recalculates your position. If you owe more, pay it as soon as you can through your usual HMRC payment method. You may also need to adjust payments on account for the following year, although this depends on the size and nature of the change.

If you have overpaid, HMRC may set the credit against another tax bill or offer a repayment. Make sure your bank details and contact information held with HMRC are current. For CIS clients, a repayment can be delayed if the deductions claimed do not match the information HMRC holds, so keep every deduction statement available.

A genuine error corrected voluntarily is generally viewed more favourably than one found later by HMRC. Penalties are not automatic simply because you amend a return, but the outcome can depend on why the mistake happened, whether reasonable care was taken and how quickly you put it right.

When it is worth asking an accountant to help

A simple missing invoice can often be amended without difficulty. It is worth getting support where the correction affects several areas of the return, changes a CIS refund, involves property income, capital gains, partnership income, dividend tax or a large change in profit.

It is also sensible to ask for help if you are amending more than one year, have received an HMRC letter, or cannot reconcile the tax calculation with your records. The objective is not just to resubmit a return. It is to make sure the revised figures are accurate, supported and tax-efficient.

At Short And Sons Accountants, we regularly help tradespeople, landlords and small business owners turn a worrying tax error into a properly documented correction. A timely amendment can protect your position, but good records and a quick check of the wider return will give you far more confidence when the next deadline comes around.