Some of the more advanced Payroll systems, can stop tax relief, during the tax year, as they check, not only the employee age, at the date of the payrun,(for age related max contribution %) but also check the pro rated max earnings cap of 115k, but in addition, the year to date amounts, so will stop any tax relief on a calendar pro rata basis. So, for example, if you are already on max avc %, and a bonus is paid out in say March, and you opt to add all of the bonus to AVC’s it will disallow any tax relief, subject to your age max relief & pro rated earnings limit also, on the bonus.
Megapay is one such system, which has these controls, which operate on a cumalative basis, so it restricts and denys tax relief to the income portion that exceeds the relief rules, but will refund tax, if later on in the tax year, the employee falls below the upper limits, eg if they reduced their AVC’s later in the year, having previously exceeded the age related limit, or annual earnings cap - on a calendar pro -rated basis.
Some other systems, have no such controls, and will allow full tax relief irrespective of any exceeding of the limits, this can be tricky as employee has a potential year end liability, but the employer also could be in hot water with the revenue, as this is strictly, against the rules. Employers have a responsibility, to check that sufficient controls are put in place.