When you pay PAYE, Class 1 National Insurance contributions, or Construction Industry Scheme and Student Loan deductions you need to give your PAYE Accounts Office reference number. Your Accounts Office reference is 13 characters long, for example 123PA00045678. You can find this on:
If you pay early, late or make more than one payment for the same month, or you are making a balancing payment for a previous tax year, you need to add four numbers to the end of your Accounts Office reference. This tells HMRC what tax year and month your payment is for. Your reference then becomes 17 characters long with no spaces between. From the 2013-14 tax year you should make balancing payments for a previous tax year using month 12 not month 13.
This checker only confirms that the characters are in the right format to allow your payment to go direct to your account. It will also add the extra four numbers to your reference number if needed.