Deep dive · Updated 08/07/2026

The most common CMS errors — and how to spot them

These failure points recur across Independent Case Examiner findings, Parliamentary and Health Service Ombudsman investigations, parliamentary committee evidence and tribunal outcomes. None of them is hypothetical; all of them are detectable from your own paperwork. The PHSO has upheld cases where CMS failings led one parent to overpay more than £8,500.

1. The wrong income, applied confidently

What happens: a stale tax year, a figure that doesn't match the P60, duplicated employments in HMRC data, or missed pension deductions. Spot it: the decision letter must state the annual figure and tax year — reconcile both against the actual HMRC records. Fix: MR with documents attached; ask the CMS to disclose exactly what HMRC returned.

2. The 25% rule misapplied — in both directions

What happens: a genuine 25%+ drop reported with evidence gets refused as "within tolerance" through comparison against the wrong baseline; or an increase is applied without the threshold being met. Spot it: ask, in writing, precisely which two weekly figures were compared. Fix: MR; the comparison is prescribed by the regulations, not discretionary.

3. Silent "assumed" shared care — and the invented agreement

What happens: unable to resolve a dispute, the CMS sets one-night-a-week assumed care — or none at all — without clearly telling the parent whose care is understated. A related pattern: the decision letter or MRN asserts that "both parties agree" a band that neither parent actually stated — a split-the-difference figure between the two accounts, dressed up as consensus. Regulation 46 requires a determination of the expected nights on the evidence, not an arbitrary compromise. Spot it: the letter is silent on the band, or records an "agreement" you never made — compare its wording against what each of you actually reported. Fix: MR stating plainly that no such agreement exists, with your care evidence attached; a contemporaneous overnight log is powerful at tribunal.

4. Arrears built on sand

What happens: arrears accrue from the initial effective date while processing drags; Direct Pay payments made and received are recorded as missed; corrected decisions are not flowed back through the arrears ledger. Spot it: demand a full arrears breakdown — every liability, every credited payment, by date. The CMS must be able to produce it. Fix: reconcile against bank records; challenge the underlying decisions, not just the total. See arrears.

5. Reported changes that vanish

What happens: a phoned-in change of circumstances is never actioned; months later the parent discovers liability continued on the old basis. Spot it: every reported change should produce a decision letter — no letter means no action. Fix: always follow calls with written confirmation; where the CMS sat on a report, seek revision back to the report date and compensation for the maladministration.

6. Variation grounds ignored or half-processed

What happens: variation applications rejected at the sift without proper reasons, unearned income evidence not pursued, or the "just and equitable" stage treated as all-or-nothing — the exact error corrected in PP v SSWP [2022] UKUT 286. Spot it: the decision letter should address each ground raised and the evidence for it. Fix: MR, then appeal — tribunals have disclosure powers the CMS rarely exercises and re-decide variations in full.

7. Phone misdirection about evidence and rights

What happens: parents are told over the phone that "only a court order or signed agreement counts" as shared-care evidence, that the other parent's account will be preferred by default, that no mandatory reconsideration is available, or that there is "no mechanism to appeal". Every one of those statements is wrong: reg 46(4) requires consideration of any agreement (the CMS's own Decision Makers Guide accepts verbal agreements — DMG 11003) and of an established 12-month pattern of care; and s.20 of the Child Support Act 1991 confers a statutory right of appeal on both parents in respect of the decision. Spot it: any phone statement that closes off evidence or challenge routes. Fix: note the date, time and number called; proceed with the MR in writing regardless; and raise the misdirection as a formal complaint — misdirection that causes you to miss a deadline strengthens both a late-MR application and a redress claim.

8. Conflating decisions with conduct

What happens: parents are told to "complain" about a wrong calculation (which burns the appeal deadline), or to "appeal" against delay and misinformation (which the tribunal cannot hear). Spot it: decisions carry MR/appeal rights; conduct is for the complaints ladder. Fix: run both tracks in parallel when both apply — they are independent and do not wait for each other.

9. Enforcement that ignores the safeguards

What happens: deductions from earnings breaching the 60% protected earnings rate; deduction orders against disputed arrears; welfare-of-the-child considerations (s.2 CSA 1991) never addressed in discretionary enforcement choices. Spot it: compare deductions against payslips; ask for the enforcement decision's reasons. Fix: the specific remedies under each power in enforcement powers — plus complaint where process was ignored.