Opening balances cover pay and tax to date. National Insurance to date is a separate record: the NIC summary, at .../employee/{employee_pk}/nic-summary/{tax_year}/, with one entry per NI category letter per tax year. Seed it at migration and the director's cumulative NI position is complete.
What does a NIC summary entry hold?
The NIable pay (niable_amount), the full split by band (ni_employee_at_lel, ni_employee_lel_to_pt, ni_employee_pt_to_uel, ni_employee_above_uel), the employee and employer totals (ni_employee_total, ni_employer_total) and an is_director flag. If the employee changed NI category part way through the year, create one entry per letter.
Where is the directorship set?
In directorship_details on the employee: is_director, the start and end dates of the directorship, and ni_calculation_method, either STANDARD or ALTERNATIVE.
Does year end need special handling?
No, once the NIC summary is seeded. Under STANDARD, each payment is assessed on cumulative annual earnings against the annual thresholds, and the seeded summary supplies what has already been deducted, so the figure corrects itself every period. Under ALTERNATIVE, NI runs period by period as for any employee, and the last payment of the tax year is recalculated on the annual basis, with the difference settled in that payment. Where a directorship began during the year, its start date pro-rates the annual thresholds.
Get started
See "National Insurance: rates, thresholds and how it is calculated" for the rules behind the calculation.