NIM05646 | Class 1 NICs: Expenses and allowances: Civic dignitaries' and Councillors’ allowances: Assessing Class 1 NICs
From HM Revenue & Customs · National Insurance Manual
If there is a regular earnings pattern, as is usually the case, the earnings period is equal to the length of the pay interval. The earnings period for a person paid monthly is therefore one month; for someone paid bimonthly it is two months and where a person is paid annually it is one year.
People who receive earnings monthly, bimonthly or annually are liable to pay Class 1 NICs monthly, bimonthly or yearly, taking into account the corresponding earnings threshold.
If expenses are paid at different intervals, the earnings period for the assessment of Class 1 NICs is the shortest interval at which any payments are made. For example, if the agreement between the LA and the councillor or civic dignitary is that
travelling expenses are paid monthly
entertainment expenses are paid quarterly, and
clothing expenses are paid annually
then the earnings period will be monthly (because this is the shortest interval at which payments of earnings are made).
Class 1 NICs will be payable if the total payments of earnings in any month reach the monthly earnings threshold. See NIM01008.
See NIM08000 for further guidance about earnings periods.