GUIDE • CHILDREN ARRANGEMENT SOLICITORS
Variation of a CMS calculation
The Child Maintenance Service (CMS) calculates child maintenance using a standard formula based on the paying parent's gross income. In most cases this produces a fair result, but the formula cannot take account of every family's circumstances. A variation allows either parent to ask the CMS to adjust the calculation where there are particular expenses or sources of income that the standard formula does not reflect. This guide explains when a variation can be sought and how the process works.
Written by Nicola Jones - Partner, Divorce and Family - Last reviewed 13th October 2026
KEY POINTS
- A variation can increase or reduce the amount of child maintenance calculated by the CMS.
- A paying parent can ask for certain "special expenses" to be taken into account.
- Either parent can ask for "additional income", such as rental income or diverted income, to be included.
- Variations are only available on specific grounds, and the CMS must consider the change just and equitable.
- If you disagree with a decision, you must ask for a mandatory reconsideration before appealing.
What is a variation?
A variation is a request to the CMS to adjust the income figure it uses to calculate child maintenance. It can reduce the paying parent's income to reflect certain expenses, or increase it to include income that the standard calculation does not capture. Either parent can apply at any time, before or after a calculation has been made. The grounds are set out in the Child Support Maintenance Calculation Regulations 2012, and a variation cannot be made for any other reason.
When can the paying parent ask for a reduction?
A paying parent can ask for "special expenses" to be deducted from their gross income. These are:
- the costs of travelling to see the child, or having the child to stay;
- costs relating to a disabled or long-term ill child who lives with them;
- repayments of certain debts taken on during the relationship for the family's benefit;
- part of the cost of boarding school fees; and
- mortgage, loan or insurance payments on the former family home, where the paying parent no longer has an interest in it.
Most special expenses must exceed £10 a week to be taken into account. Receiving parents cannot apply for a special expenses variation.
When can the receiving parent ask for an increase?
Either parent can ask for "additional income" to be included in the calculation. This covers:
- unearned income of at least £2,500 a year, such as rent, dividends or interest;
- earned income of at least £100 a week, where the paying parent receives benefits but also has income from work or a pension; and
- diverted income, where the paying parent appears to be reducing their income, for example by paying it to a new partner or retaining it in a company they control.
The CMS only takes into account gross income up to £3,000 a week. Where the paying parent earns more, the receiving parent will need to apply to court for top-up maintenance.
If you think a child maintenance calculation does not reflect the true position, click below to speak to one of our family solicitors in confidence.
How do I apply and what happens next?
You can apply through your online CMS account or by telephone, setting out the grounds and providing supporting evidence, such as travel receipts, loan statements or details of the other parent's investments or business. The CMS will usually contact the other parent for their response. Even where a ground is established, the CMS will only agree to a variation if it considers it just and equitable to do so, taking into account the welfare of the child. Variations are not available in some cases, for example where the paying parent receives certain benefits.
What if my income changes instead?
A change in income is not a ground for a variation. The CMS reviews each calculation annually using income figures from HMRC. Between reviews, either parent can report a change in the paying parent's income, but the CMS will only recalculate if the income has changed by at least 25%. Other changes, such as a change in the number of nights the child stays with each parent or the paying parent having another child, should also be reported.
What if I disagree with the decision?
If you disagree with a CMS decision, including a decision on a variation, you must first ask the CMS to look at it again. This is called a mandatory reconsideration. If you still disagree after receiving the outcome, you can appeal to the First-tier Tribunal (Social Security and Child Support), which is independent of the CMS. Strict time limits apply, so it is important to act promptly.
Common questions
A father with parental responsibility has the same say as the mother in major decisions about the child. There’s no automatic right to equal time, but the court starts from the position that a child benefits from both parents being involved, unless that would put the child at risk.
Cafcass is the Children and Family Court Advisory and Support Service, which represents children’s interests in family court cases in England. It carries out safeguarding checks and, where needed, prepares reports on the children’s welfare and wishes. In Wales, the same role is carried out by Cafcass Cymru.
IN THIS GUIDE
When can the paying parent ask for a reduction?
When can the receiving parent ask for an increase?
How do I apply and what happens next?
What if my income changes instead?
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