Child Maintenance Variation or Revision in the United Kingdom
Reviewed by the Commoner Law Editorial Team. 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.
Common Questions
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.