Paul B

Hi - I recently submitted my self assessment for 22/23 and have been told I underpaid - this I accept as I earned slightly over the child benefit allowance. HMRC now want a payment of £1000+. However, in the tax calculation HMRC are using the standard allowance of £12,570, but for the entire period I was under tax code 1383M with an allowance of £13,830. That's a significant difference. I called and questioned this and was told 'that doesn't matter' - I can't understand this, surely my tax-free allowance should be measured against the tax code that applied at the time? Any advice on how to appeal the underpayment and get the allowance/code issue recognised please?