Reviewing a Self Assessment calculation

Reviewing a Self Assessment calculation means comparing the figures and reliefs with the approved return and source records. Distinguish a calculation error from a payment allocation issue or a later HMRC adjustment before choosing the correction or challenge route.

Check the tax year, income categories and tax already deducted. Ask for an explanation of material differences rather than comparing only the final balance with last year.

Separate the tax calculation from the account statement

Obtain both the computation for the tax year and the Self Assessment statement. HMRC distinguishes the calculation of annual tax from the statement showing amounts owed and payments made. ^[1] A difference between the two documents is not automatically an error. The statement may include account movements that are not part of the calculation of that year's income liability.

Label each document with its tax year, issue or download date and whether it reflects an original or amended return. Keep earlier versions where changes have occurred. A useful review starts by identifying which figures are being compared, rather than comparing an old computation with a current account balance and assuming the unexplained difference must be extra tax charged on the same income.

Reconcile income and deductions to source records

Check employment pay and tax against P60s, P45s and relevant benefit information. Compare pension, savings, dividend and property figures with their supporting schedules. Each category should have an explained source and should appear only once. Where a provider has issued a corrected statement, confirm that the revised figure has reached the return rather than remaining only in the supporting folder.

Review amounts described as net or gross carefully, especially pension contributions, Gift Aid donations and income received after deductions. A figure can be arithmetically correct but entered on the wrong basis. Keep the preparer's explanation of any adjustment, and ask about values that differ from the source document. Do not replace a calculated figure with a bank receipt without understanding why the two amounts differ.

Check status, year and relief assumptions

Confirm the tax year, relevant residence status and any Scottish or Welsh taxpayer information used. Identify claims, losses or allowances that depend on supporting conditions. The review does not require guessing every technical calculation yourself, but it should establish that the inputs reflect your circumstances and that material assumptions are visible. An unexplained tick box can affect the outcome as much as a mistyped income figure.

If an estimate from an earlier adviser or online calculator differs, compare its assumptions with the return. Check whether it omitted benefits, investment income or a pension event. Keep a list of the differences and resolve them one by one. Self Assessment for high earners explains why a complete income and event inventory is useful when employment earnings are only one part of a more complex personal tax position.

Reconcile payments and amounts still due

Use the statement to match payments already made, credits, repayments and amounts now outstanding. HMRC's statement guidance explains that it can include items such as payments on account, penalties and interest alongside the balance to pay. ^[2] Distinguish tax for the completed year from amounts relating to another period. This avoids treating a payment towards the following year as additional tax on the same year's income.

For each payment, retain the date, amount and reference, then check its allocation. If money left the bank but does not appear on the account, investigate with the payment evidence. If a credit has been moved or repaid, identify that transaction rather than counting it as still available. A clear reconciliation should explain the path from the annual calculation to the current amount payable.

Resolve discrepancies through the appropriate route

If the return contains an error, consider the correct amendment process and any deadline. If the return is accurate but the account allocation appears wrong, focus the enquiry on that payment or credit. A calculation review, account query and appeal against a formal decision are different tasks. Identify which problem exists before sending a broad disagreement that does not explain what HMRC should check.

For help reviewing the return and statement together, see Tax penalty appeal preparation. Provide the computation, submitted return, current statement and supporting evidence for the item questioned. Ask for a reconciliation that distinguishes confirmed figures from unresolved assumptions. Keep the outcome and any revised calculation with the file so subsequent payments are based on the reviewed position rather than the first estimate encountered.

Before approval, note the amount due, deadline, payment reference and any expected future instalment separately. If a revised return is being processed, identify which figures are provisional and when the account will be checked again. This makes the next action clear without assuming that a submitted correction has already altered every statement or payment reminder.

Illustrative scenario

A taxpayer expects a refund but sees a balance due. Reconciliation shows that a payroll deduction was entered incorrectly, allowing the adviser to assess the appropriate return correction.

Preparation checklist

• Confirm the tax year
• Compare income and reliefs
• Reconcile deductions
• Identify the correct response route