Challenging a decision from the Child Maintenance Service
Last updated: 28/07/2026
If you think your child maintenance calculation is wrong you can ask for a mandatory reconsideration, 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?
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 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 mandatory reconsideration you could then go on to ask for a supersession or a variation
Details of how to ask for a mandatory reconsideration will be included in the letter you received about the calculation.
How do I ask for a mandatory reconsideration?
You must ask for your mandatory reconsideration 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 mandatory reconsideration. However, you will need to tell them why your request is late. You don’t need proof for why it’s late, but you should attach your reason with your form requesting the mandatory reconsideration.
The CMS have the right to reject your mandatory reconsideration 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.
How to do it
You can ask for a mandatory reconsideration by:
- calling CMS on 0800 171 2345
- writing to CMS
- contacting CMS in your online account if you have one
What you need
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
Who to contact
Contact:
Child Maintenance Service
Child Maintenance Service 21
Mail Handling Site A
Wolverhampton
WV98 2BU
What to do if your circumstances change
| Topic | What it is | When you use it | How to ask | Key points / requirements |
|---|---|---|---|---|
| Supersession | A new child maintenance calculation when circumstances change. | • Your child stops being classed as a child • You, your child or the paying parent move to a new country • Your child goes into foster care • You stop being responsible for the child’s day-to-day care • Your child passes away • The paying parent passes away |
Call CMS on 0800 171 2345 or use your CMS online account. | • You can be fined up to £1,000 if you don’t report required changes promptly. • New calculation can be backdated to the date of change. |
| Variation (receiving parent) | A change to the income used in the calculation if you think it’s wrong. | When you believe the paying parent has undeclared income or other income not included in the calculation. | • Contact CMS by phone or in writing. • Use your CMS online account (if you have one). • If CMS asks for more info, you must send it within 14 days. |
• If CMS accepts, they tell the paying parent, who has 14 days to disagree. • If they disagree, you have 14 days to respond. |
| Variation (paying parent) | A change to reduce payments if certain expenses are taken into account. | When you have specific extra costs that affect your ability to pay. | Contact CMS by phone or in writing, or use your CMS online account. | Contact costs, long‑term disability/illness costs of a “relevant child”, debts from the relationship and boarding school fees may be considered. |
| Appeal | A formal challenge to a CMS decision, decided by an independent tribunal. | When you’re unhappy with a CMS decision (e.g. on a supersession or variation) after mandatory reconsideration. | Follow the instructions in the mandatory reconsideration letter; you must appeal within one month. | Important: • You must first ask CMS for a mandatory reconsideration. • Appeal is not for delays or poor service — use the complaints process instead. • You may need a solicitor or legal help, which can be expensive. |
| Postal contact for CMS | Address for written requests (e.g. variations). | When writing by post instead of calling or using your online account. | Child Maintenance Service 21 Mail Handling Site A Wolverhampton WV98 2BU |
• Ask the Post Office for proof of postage. • Keep a copy of any letter or notes of what you told CMS. |