Amending a Company Tax Return

When an error is found in a Company Tax Return, identify the original submission, period and affected figures. HMRC generally allows amendments within twelve months of the filing deadline. Outside that window, the appropriate route depends on the circumstances and should be assessed promptly.

Explain what changed and why, with corrected accounts or computations where needed. Check the effect on payment, interest, claims and other periods. Do not replace the internal PDF and assume HMRC has received the correction; retain evidence of the actual amendment or other submission.

Identify the error and preserve the original submission

Locate the accepted return, computation, accounts and receipt for the relevant company and period. Describe the error, how it was discovered and the evidence supporting correction. Do not overwrite the original file before preserving it. A clear history helps distinguish a genuine correction from a later change in assumptions and shows which version HMRC actually received.

HMRC generally permits amendments within twelve months of the filing deadline, with different routes needing consideration outside that window. ^[1] Check the actual dates promptly rather than assuming twelve months from discovery or submission. If a deadline is close, identify the immediate procedural question with the adviser while continuing to assemble the supporting calculation.

Recalculate the connected figures

Review the effect on taxable profit, liability, claims and losses. An error in one period can affect later brought-forward amounts or another company's position. Prepare a reconciliation from the original figures to the corrected version with reasons. Do not simply change the final tax box while leaving supporting schedules inconsistent.

Consider whether the underlying accounts also need correction and what separate reporting steps may follow. A tax amendment and a Companies House accounts change are not automatically the same process. Use Company tax returns and statutory accounts to keep those obligations distinct and Losses in a limited company where loss movements affect later periods. Identify each required action rather than assuming one uploaded document updates every related record.

Choose the correct submission route

Use the current method appropriate to the amendment and circumstances. For returns associated with the former joint service, HMRC's closure guidance explains the post-closure position. ^[2] Do not tell staff to reopen a service that ended in March 2026. Confirm the software or adviser can submit the required amendment for the relevant period and preserve evidence of acceptance.

Where the ordinary amendment window has passed, obtain advice on the appropriate claim, disclosure or other route. Do not assume an email with a corrected PDF has the same effect as a valid statutory amendment. Keep the facts and desired correction clear while the adviser assesses procedure, timing and any related consequences.

Address payment and conduct transparently

If additional tax is due, review payment, interest and any penalty or disclosure questions promptly. If the correction reduces tax, identify the repayment or account adjustment process. Do not assume the bank position changes immediately when an amendment is accepted. Keep the payment reference and allocation connected to the corrected period.

Explain errors honestly and retain supporting evidence. Never alter invoices, dates or records to conceal why the original return was wrong. A good-faith correction still needs accurate facts and appropriate procedure. If HMRC is already checking the period, coordinate the response with the adviser and the existing correspondence rather than submitting disconnected explanations through several routes.

For Corporation Tax return support, provide the original pack, error summary, corrected calculations and key dates. Ask for a defined amendment plan and any effects on other periods or filings. After submission, retain both versions, approval and acceptance evidence. Update the loss, asset or other continuing schedules so next year's return starts from the corrected position and does not silently reintroduce the same mistake.

Document the correction for future preparers

Write a concise explanation of what was wrong, how it was discovered and which figures changed. Link the explanation to the original and corrected calculations without overwriting the historical submission. If the correction affects a brought-forward balance, loss schedule or accounting entry, identify that consequence explicitly for the next year's preparer.

Keep any payment adjustment, correspondence and submission acknowledgement with the correction record. Check whether an agent change or software migration has left another copy of the old figures in active use. The company should be able to answer a later question about the difference between returns without reconstructing the entire investigation. This also helps distinguish a deliberate, documented correction from an unexplained change that could cause the same error to reappear when old working papers are reused.

Illustrative scenario

A company discovers that an equipment disposal was omitted from its tax computation. The accountant reviews the accounts and allowance position, prepares the correction and checks the amendment route. The company retains the original and revised submissions with an explanation, then reviews any additional payment due.

Preparation checklist

• Identify the error and original filing details.
• Check the amendment deadline and appropriate route.
• Prepare supported corrected figures and explanations.
• Confirm submission, payment and related-period consequences.