Tool · Updated 07/07/2026

Check your case: the fairness self-assessment

Work through the twelve checks below with your most recent decision letter (and ideally your P60 or tax return) in front of you. Answer honestly. At the end you get one of two results: the assessment stands up — in which case the fair and lawful course is to pay it — or specific potential faults, each with the exact route to fix it.

Before you start

This tool checks the CMS's work, not whether the law is fair. It cannot see your documents, so treat the result as a structured starting point, not a verdict. Nothing here is legal advice.

1. Does the annual gross income on your decision letter match the paying parent's P60 or self-assessment for the tax year the letter cites?

Compare the exact figures. The letter must state the tax year used.

Yes, it matches No, or it doesn't state the year Don't know yet

2. Is the tax year used the latest complete one HMRC could have supplied?

A calculation made after summer should rarely rest on a year ending two or more Aprils ago.

Yes No, an older year was used Don't know

3. Have all pension contributions been deducted from the income figure?

Occupational scheme contributions are usually already reflected; personal/relief-at-source ones must be claimed with evidence.

Yes / no pension contributions exist No, contributions were ignored Don't know

4. Has the paying parent's income changed by 25% or more since the tax year used?

No — within 25% Yes — and the CMS is still using the old figure Don't know

5. Is the correct rate band applied (nil / flat £7 / reduced / basic / basic plus), with the split at £800 handled correctly?

Check against the calculation guide: 12/16/19% up to £800 a week, then 9/12/15% above it.

Yes, the maths reproduces No, I get a different number Haven't run it

6. If the paying parent has other children living with them, was gross income reduced by 11% / 14% / 16% first?

Yes / no other children No, the reduction is missing or wrong Don't know

7. Is the shared care band right for the actual overnight pattern?

52–103 nights = 1/7 off; 104–155 = 2/7; 156–174 = 3/7; 175+ = half plus £7 per child. See shared care.

Yes / no overnight care No, the band is wrong or care was ignored Don't know what band was used

8. Is the number of qualifying children right, and does each still qualify?

Liability normally ends at 16, or up to 20 in full-time non-advanced education while Child Benefit remains payable.

Yes No

9. Are you on a default maintenance decision (£39/£51/£64 a week) even though income information was provided?

No / not on a default Yes — a default persists despite information being supplied

10. Do the arrears (if any) reconcile against your own payment records?

Yes / no arrears No — the arrears figure doesn't match my records Never been given a breakdown

11. Is there material income the calculation ignores — dividends, rental income, or assets over £31,250 — on either side?

Relevant to receiving parents considering a variation, and to paying parents anticipating one.

No Yes

12. Are you within one month of a decision letter you disagree with?

No / I agree with the decisions Yes No — the decision I dispute is older than a month

Sources

This tool applies the rules documented across the site — see the source tables on the calculation, income & HMRC, shared care and mandatory reconsideration, each citing the Child Support Act 1991, the CSMC Regulations 2012 and current GOV.UK guidance.