When tax rules change at the start of a year, the first payroll run under the new rules tends to expose problems that pre-date them. December is the window to find those.
Reconcile the year before it closes
Compare total gross, contributions and tax withheld across the year against what was declared and paid. Differences are far easier to explain and correct now than after the year has closed and the rules have changed underneath them.
Include the items processed outside normal payroll: bonuses, settlements, benefits in kind, anything paid by exception. These are where reconciliations usually break.
Check the data the calculation depends on
The new rules will be applied to whatever your records hold. Verify that employee identifiers, start dates, contract types, gross amounts and any exemptions are current.
Pay particular attention to staff whose circumstances changed mid-year — a promotion, a move from part-time, a second contract. Records for these employees are the most likely to be stale.
Test the new rules before the live run
Once the payroll system is updated, run the January calculation in parallel against a sample of employees spanning the range: near the minimum wage, in the middle, above any threshold or ceiling. Check the result by hand.
An error found in a test run is an afternoon. The same error found in a live run is a correction, a set of conversations with staff, and possibly an amended declaration.
Prepare the explanation
Where net pay changes for the same gross, tell people before they see the payslip. A short note explaining what changed and why prevents a week of individual questions and the impression that something went wrong.
Confirm the deadlines
Declaration and payment dates may shift around the year end. Confirm them rather than assuming continuity, and diarise them.