While collecting together the papers for my self-employed accounts to 30 November 2009, I noticed £1,500 of sales should have been recorded in the accounts to 30 November 2008, but were missed from that year. Should I add those old sales receipts to the 2009 sales and declare the total in my 2009/10 tax return?
The correct approach is to amend your 2008/09 tax return with the increased sales figure for the 2008 accounts, so the extra income falls in the 2008/09 tax year. This adjustment will increase the tax due for 2008/09 and you will have to pay some interest on the late paid tax. If you supply the Taxman with a full explanation of the error, without being asked to do so, you will probably get away with a zero penalty.
If, as you suggest, you add the missing 2008 income to your 2009 accounts and include the total in your 2009/10 tax return you will pay approximately the right amount of tax overall, but for the wrong tax years. This mis-timing of tax payments can attract penalties as you need to pay the correct amount of tax at the right time. Both your 2008 and 2009 accounts will be technically incorrect. You would need to declare the adjustment to your 2009 accounts on your 2009/10 tax return. If you don’t make a full disclosure of the error on your 2009/10 tax return and the Taxman discovers the ‘fix’, he may conclude you have deliberately concealed the error and impose a penalty of up to 100% of the tax underpaid for 2008/09.