CMS Variation or Revision (2026 Legal Guide) — Rules & Requirements
About this article
Sourced from UK Acts of Parliament, statutory instruments, and official guidance. Written in plain language for general understanding — this is educational content, not legal advice. Our editorial standards
What is this right?
The Child Support Act 1991 sets out two routes to change a CMS calculation: revision (s.16, within 30 days, for errors in the original calculation), and variation (s.28A, anytime, for special expenses or additional income that the standard gross-income method doesn't capture). About 800,000 paying parents are on a CMS calculation, and Gingerbread reports that enforcement against non-paying parents is the single biggest gap in the system — but for receiving parents the variation route is the under-used lever, and for paying parents it's revision.
Variation grounds that work most often:
- Special expenses (paying parent) — contact costs above £15/wk, prior debt payments, costs of a disabled child, school fees.
- Additional income (receiving parent) — assets above £65,000, unearned income, diversion of income through a controlled company.
- Lifestyle inconsistent with declared income — the paying parent's declared income is implausibly low given visible spending.
When does it apply?
- You're a paying or receiving parent on a CMS calculation and the figure is wrong.
- For revision under s.16 — within 30 days of the calculation, or up to 13 months late with reasons.
- For variation under s.28A — anytime, but only takes effect prospectively (and one month before the application).
What to Do If Your CMS Calculation Is Wrong
- Apply via the CMS account online or call 0800 171 2345. Send a written variation application by post if the issue is documentary (asset valuations, company accounts, school invoices).
- For a variation, identify the specific ground and the supporting evidence — the CMS won't go fishing for evidence on your behalf.
- For a revision (errors), point to the specific figure that's wrong and provide the correct one with documentation.
- If CMS refuses, you have 30 days to appeal to HMCTS (SSCS).
What should you NOT do?
- Don't conflate revision with variation — they have different time limits and different grounds.
- Don't expect a variation to apply retrospectively — variations only run forward, plus the month before the application.
About Benefits Rights in United Kingdom
DWP refuses about three in four PIP applications first time. Around 58% of appeals are overturned at tribunal. The gap is procedural: people miss the 1-month Mandatory Reconsideration deadline, or stop after MR because nobody told them the tribunal win-rate is ~58%. Section 9 of the Social Security Act 1998 is the master statute — combined with the Social Security and Child Support (Decisions and Appeals) Regulations 1999, it gives every claimant a right to challenge any benefits decision in writing.
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Common Questions
What is the child maintenance variation or revision right in United Kingdom?
The Child Support Act 1991 sets out two routes to change a CMS calculation: revision (s.16, within 30 days, for errors in the original calculation), and variation (s.28A, anytime, for special expenses or additional income that the standard gross-income method doesn't capture). About 800,000 paying parents are on a CMS calculation, and Gingerbread reports that enforcement against non-paying parents is the single biggest gap in the system — but for receiving parents the variation route is the under-used lever, and for paying parents it's revision.Variation grounds that work most often:Special...
When does child maintenance variation or revision apply?
You're a paying or receiving parent on a CMS calculation and the figure is wrong.For revision under s.16 — within 30 days of the calculation, or up to 13 months late with reasons.For variation under s.28A — anytime, but only takes effect prospectively (and one month before the application).
What should I do if the CMS has calculated child maintenance wrongly?
Apply via the CMS account online or call 0800 171 2345. Send a written variation application by post if the issue is documentary (asset valuations, company accounts, school invoices).For a variation, identify the specific ground and the supporting evidence — the CMS won't go fishing for evidence on your behalf.For a revision (errors), point to the specific figure that's wrong and provide the correct one with documentation.If CMS refuses, you have 30 days to appeal to HMCTS (SSCS).
What mistakes should I avoid with child maintenance variation or revision?
Don't conflate revision with variation — they have different time limits and different grounds.Don't expect a variation to apply retrospectively — variations only run forward, plus the month before the application.