Guide · Updated 07/07/2026

The annual review

Every CMS case is reviewed once a year, on the anniversary of the initial effective date. The CMS re-pulls income from HMRC and re-runs the calculation. It is routine — and it is also the moment when errors most often enter a previously stable case.

What actually happens

1. About 30 days before your review date

   The CMS requests the latest available complete tax-year income for the paying parent from HMRC and issues both parents an annual review letter setting out the proposed new figures.

2. The new HMRC figure is applied

   Important nuance: at annual review the fresh HMRC figure is applied without the 25% tolerance test — that test governs in-year changes, not the review itself. However, if the case has been running on a current income figure, the CMS compares the latest HMRC figure against it and the 25% rule shapes whether the basis switches back to historic income.

3. Both parents get 14 days (in practice, until the review date) to respond

   Check the letter and report anything wrong or out of date: pension contributions not reflected, shared care changes, other children, income already 25%+ different from the HMRC year quoted.

4. The new calculation takes effect from the review date

   It runs for the next year unless a reported change or variation supersedes it.

Why figures jump at review

• A different tax year lands. Last year's calculation may have used 2023/24; this year's uses 2024/25 with a bonus, overtime or a one-off in it.
• A current-income basis quietly reverts to historic. If you reported a drop mid-year, the review can pull the old, higher HMRC year back in unless the drop persists and is re-evidenced.
• Pension contributions vanish. Relief-at-source contributions the CMS previously deducted are not carried over automatically — re-evidence them.
• Data mismatches. Payroll errors, duplicated employments, or HMRC records containing income that is not the paying parent's (it happens) flow straight into the calculation unchecked.