This was a first for me, but I expect it heralds the first of a flood.
Time was, when you would file an SA return with a degree of certainty that the figure filed would correspond with the amount that gets posted to the SA record. Included in these returns would be those for certain individuals who had both employment and self employment sources, with sufficient income from all sources such that in isolation the calculations of Class 1, 2 and 4 would exceed the combined annual total liability. In those circumstances you could either apply in advance for deferral of Class 2 & 4, that would be separately assessed, or you could apply in arrears to the NI office for an NIC assessment for the year that would give rise to a refund. Either way, the SA return was fixed and certain.
I have just come across a case where there was no application for deferral, and we were gearing up to applying for the NI to be assessed and refunded, only to get preempted by an amendment to the SA return under S.9ZB(1) TMA 1970 showing a reduced class 4 NIC charge (with an explanation in principle but no supporting calculation).
Irritatingly, our tax software (Sage) has a box for entering primary class 1 NIC paid against employment income sources, but it makes no use of these figures in working out the SA calc. The NI assessment is not trivial to do manually, and if you don't have the figures to hand when completing the SA return, HMRC will helpfully provide you with pay and tax and BIK on enquiry, but not the NIC
Anyone else getting these? If not, consider this a heads up. You likely will presently.
With kind regards