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HMRC internal manual

National Insurance Manual

Class 1A National Insurance contributions: Calculating Class 1A NICs: Using the P11D(b) adjustment facility where benefit reported on P11D in a non Class 1A NICs box

So that any benefit reported on the P11D, in a box which has no Class 1A NICs indicator, is included in the necessary Class 1A NICs calculation, employers need to use the adjustment facility provided at section 4 of the P11D(b). This allows any cash equivalent not included in a Class 1A P11D box to be added to the total figure on which an employer calculates his Class 1A NICs liability.

The P11D(b) adjustment facility can also be used where a benefit is reported in a box which has a Class 1A NICs indicator but for which no Class 1A NICs are payable, see NIM15300.