Challenging a decision from the Child Maintenance Service
Last updated: 10/04/2025
If you think your child maintenance calculation is wrong you can ask for a revision, supersession, or variation. You can also appeal to have one of these decisions re-examined. Which of these you ask for depends on what you think is wrong.
CMS Contact Details
Child Maintenance Service 21
Mail Handling Site A
Wolverhampton
WV98 2BU
Telephone: 0800 171 2345
Relay UK: 18001 then 0800 171 2345
Monday to Friday, 8.30am to 3.30pm
Calls are free from mobiles and landlines.
What to do if the CMS made a mistake?
Revision or Mandatory Reconsideration
If you think the Child Maintenance Service has made a mistake in your child maintenance calculation you can ask them to look at it again. This is called a revision or a mandatory reconsideration.
- the mistake could be because the CMS got the number of children wrong or did not use the paying parent’s correct income
- depending on what is found in the revision you could then go on to ask for a supersession or a variation
Details of how to ask for a revision will be included in the letter you received about the calculation.
What to do if your circumstances change?
Supersession
If there is a change in circumstances after your child maintenance calculation has been made you can ask for a supersession.
There are certain things that you’re legally required to tell the CMS about, and if you don’t tell them you could be fined up to £1,000. You don’t have to tell CMS about the change until after it has happened, but it’s important that you tell them as quickly as possible to avoid the fine.
You have to tell the CMS if:
- your child stops being classed as a child, e.g. because they leave approved education
- you, your child and/or the paying parent move to a different country
- your child gets put into foster care or adopted
- you stop being responsible for the day-to-day care of the child you get child maintenance payments for
- your child passes away
- the paying parent passes away
To apply for a supersession contact the Child Maintenance Service by phone or in writing. Once you’re supersession has been approved, the new calculation can be backdated to the time of the change.
What to do if you think the paying parent hasn’t declared all of their income to the CMS?
Variation
If you are the parent receiving child maintenance you can apply for a variation if you think the income used in the calculation is wrong. If you believe the parent paying maintenance has income they have not declared you can ask that the Child Maintenance Service take this into account. The Child Maintenance Service need proof of this other income before they can make the variation. Unfortunately this can be difficult to get in some circumstances.
If you are the parent paying maintenance you can also ask for a variation. Certain expenses can be taken into account and could reduce the amount of child maintenance you have to pay. These include:
- contact costs
- costs of a long-term disability or illness of a “relevant child”
- debts of the relationship
- boarding school fees
Contact the Child Maintenance Service by phone or in writing to apply for a variation. Give as much information as you can about the other parent’s income and keep a note of what you have said or a copy of the letter you have sent.
If the CMS accepts, they tell the paying parent about the decision. The paying parent then has 14 days to say if they disagree. In some cases, the paying parent will then also ask for a variation – find out more about this on Citizen’s Advice.
If the paying parent disagrees with the decision, you will then have 14 days to respond to anything they’ve said.
What to do when you’re unhappy with a CMS decision?
Appeals
If you are unhappy with a decision the Child Maintenance Service has made, on a supersession for example, you can appeal.
- an appeal is carried out by a separate tribunal which is not part of the Child Maintenance Service
You must first ask the CMS to look at the decision again (revision) but if you are still not happy you can submit an appeal.
How do I challenge a decision from the CMS?
This is the process that needs to be followed to make sure your complaint is dealt with correctly.
How do I ask for a revision?
You must ask for your revision within one month of the date on your decision letter from CMS.
If over a month has passed since you got your letter, it is still worth asking for a revision. However, you will need to tell them why your request is late. You don’t need proof for the reason why it’s late, but you should attach your reason with your form requesting the revision.
The CMS have the right to reject your revision if it’s late. If you disagree with the rejection, you can appeal this through a tribunal if you are still within 13 months of the date on your CMS decision letter.
You can ask for a revision by:
- calling CMS on 0800 171 2345
- writing to CMS
- contacting CMS in your online account if you have one
What you need:
- 12-digit reference number on the letter which says how much child maintenance you are getting (if you don’t have a paper copy or scan, check in your online account)
- if you can’t find your reference number, you need to give your name and address, and the name and date of birth of the child you’re paying the maintenance for
- if you send the CMS a letter by post, you should ask the Post Office for proof of the postage
- if you call CMS, make notes about what time and date you called, the person you spoke to on the phone and what they said
- if you call CMS, after you come off the phone, send them a letter or email with the above info so you have what happened in writing
- if you can show that the calculation is wrong, you should send a copy of your workings to the CMS
Contact:
Child Maintenance Service
Child Maintenance Service 21
Mail Handling Site A
Wolverhampton
WV98 2BU
How do I ask for a supersession?
You can ask for a supersession at any time.
You can ask for a supersession by:
- calling CMS on 0800 171 2345
- contacting CMS in your online account if you have one
What you need:
- 12-digit reference number on the letter which says how much child maintenance you are getting (if you don’t have a paper copy or scan, check in your online account)
- if you can’t find your reference number, you need to give your name and address, and the name and date of birth of the child you’re paying the maintenance for
- if you call CMS, make notes about what time and date you called, the person you spoke to on the phone and what they said
- if you call CMS, after you come off the phone, send them a letter or email with the above info so you have what happened in writing
How do I ask for a variation?
You can ask for a variation at any time.
You can ask for a variation by:
- writing to CMS
- contacting CMS in your online account if you have one
What you need:
- 12-digit reference number on the letter which says how much child maintenance you are getting (if you don’t have a paper copy or scan, check in your online account)
- if you can’t find your reference number, you need to give your name and address, and the name and date of birth of the child you’re paying the maintenance for
- if you send the CMS a letter by post, you should ask the Post Office for proof of the postage
- give the CMS as much information about the paying parent’s income or assets if they ask about this. The CMS then has to ask HM Revenue and Customs (HMRC) for more information, but this will only be available if the paying parent reported their income or assets to them
- the CMS might ask you for more information – you must then send it to them within 14 days
Contact:
Child Maintenance Service
Child Maintenance Service 21
Mail Handling Site A
Wolverhampton
WV98 2BU
How do I ask for an appeal?
You must appeal within one month of receiving the result of your revision/mandatory reconsideration.
Details of how to appeal will be in the letter you receive with the result of the mandatory reconsideration. You should not submit an appeal for something like a delay in payment, or if you received poor service. You should submit a complaint in cases like this. Find out more about how to complain about the CMS.
Think carefully about what you’re going to do, as you may need help from a solicitor or other legal service to submit an appeal.
Find out more about appeals on the Government website.