A practical guide to the laws governing Child Maintenance Service calculations, revisions, supersessions, variations, shared care, income, pensions, unearned income, evidence and appeals.
If you believe CMS has used the wrong income, ignored evidence, calculated shared care incorrectly, failed to deduct pension contributions, added income incorrectly or made another decision that does not match the legislation, this guide helps you identify the law that may apply and the route for asking for the decision to be corrected.
The legal framework
The statutory child maintenance system does not operate simply on CMS policy. Its powers and calculation rules come from Acts of Parliament and regulations made under those Acts.
Child Support Act 1991
This is the principal Act behind the statutory child maintenance scheme. It covers calculations, information, revision, supersession, variations, appeals, collection and enforcement.
Read the 1991 Act2012 Calculation Regulations
These regulations contain much of the detailed law used by the current statutory scheme, including income, the 25% test, current income, shared care and variations.
Read the 2012 RegulationsThe four routes people should understand
Revision
Used where an existing decision can legally be reconsidered and revised. This is closely connected to mandatory reconsideration.
Main law: Child Support Act 1991, section 16.
Supersession
Generally used where an existing decision needs replacing, for example because circumstances have changed or another statutory ground for supersession applies.
Main law: Child Support Act 1991, section 17.
Variation
Allows specified income or expenses outside the ordinary calculation rules to be taken into account where the statutory variation conditions are satisfied.
Main law: sections 28A–28G and Schedule 4B.
Tribunal appeal
If CMS does not correct an appealable decision following the required reconsideration process, a person may be able to appeal to the independent First-tier Tribunal.
Main law: Child Support Act 1991, section 20.
Key sections of the Child Support Act 1991
Welfare of children
When exercising discretionary powers under the Act, the Secretary of State must have regard to the welfare of any child likely to be affected.
This can matter where CMS is making a discretionary decision rather than simply carrying out a fixed mathematical rule.
Official lawMaintenance calculations
Section 11 is the core statutory power for making a maintenance calculation. The amount is determined in accordance with Schedule 1 and regulations made under the Act.
If you dispute the figure, ask CMS to identify the precise income, calculation method and legal provisions used.
Official lawDefault maintenance decisions
CMS can make a default maintenance decision in circumstances where it does not have sufficient information to make the normal calculation.
A default figure should not automatically be treated as proof of the person's actual income.
Official lawInformation and evidence
Section 14 allows regulations requiring relevant people to provide information or evidence needed for decisions, calculations, collection and enforcement.
This is important because decisions should be based on the information required by the statutory scheme, rather than unsupported assumptions.
Official lawRevision of decisions
This is one of the most important provisions when a person says CMS made the original decision incorrectly.
A qualifying decision can be revised either following an application or, in appropriate circumstances, on the Secretary of State's own initiative.
A revision will normally replace the original decision and, subject to the detailed rules, can take effect from the date the original decision took effect.
Section 16Superseding an earlier decision
Section 17 permits certain existing maintenance decisions to be replaced by a new decision.
This is particularly important where circumstances have changed after the previous decision.
Appeals
Section 20 provides the statutory appeal route for appealable child-maintenance decisions.
In most ordinary CMS calculation disputes, the person must first go through mandatory reconsideration before appealing to the First-tier Tribunal.
Section 20Correction of accidental errors
Section 28ZD permits regulations dealing with correction of accidental errors in decisions or records of decisions.
This is different from using revision to challenge the substance of a calculation.
Section 28ZDVariations — Sections 28A to 28G
Applying for a variation
Allows an eligible person to ask for the ordinary rules used to calculate maintenance to be varied.
The applicant must identify the ground on which the variation is sought.
Section 28APreliminary consideration
Deals with CMS's initial consideration of an application for variation before the application proceeds further.
Section 28BRegular payments condition
Contains provisions concerning regular payments in certain variation applications made by the non-resident parent.
Section 28CDetermination of application
Deals with the decision on the variation and, in relevant circumstances, consideration of additional information and evidence.
Section 28DMatters CMS must take into account
Section 28E sets out principles that CMS must consider when deciding whether to agree to a variation.
These include the principle that parents should be responsible for maintaining their children when they can afford to do so.
CMS must also take account of representations made by the relevant parties.
Agreement to a variation
Section 28F allows CMS to agree to a variation where the case falls within an authorised variation ground and CMS considers the variation just and equitable in all the circumstances.
The welfare of any child likely to be affected is expressly relevant to this decision.
Variation of an existing calculation
This is particularly important where a maintenance calculation is already in force.
Section 28G links variation applications with the statutory revision and supersession machinery.
Section 28GGrounds for variations
Schedule 4B is central to understanding which types of expenses and additional income can legally form the basis of a variation.
Schedule 4BWrong income — one of the most important areas
Many disputes arise because the income used by CMS does not match what the paying parent believes they actually earned.
The starting point is to establish exactly which type of income CMS legally used.
Historic income or current income?
Regulation 34 of the Child Support Maintenance Calculation Regulations 2012 provides the general rule for determining gross weekly income.
The normal starting point is historic income.
Current income is relevant where the statutory conditions are met, including where current income differs from historic income by at least 25%.
A person's income being different from the HMRC historic figure does not automatically mean CMS must immediately replace it. The statutory 25% test and other exceptions in regulation 34 must be considered.
Historic income
Sets out how historic income is determined using the HMRC figure and how it is converted into weekly income.
Regulation 35The HMRC figure
Defines the income information forming the HMRC figure for the calculation, including specified employment, pension, taxable benefit and trading income.
It is important to compare the actual HMRC data against the figure appearing in the CMS calculation.
Regulation 36Current income
Current income can include income as an employee or office-holder, self-employment and pension income, subject to the detailed rules.
Regulation 37Employment and irregular earnings
Regulation 38 contains detailed rules for current employee income.
Where earnings are regular, CMS can use the weekly equivalent. Where earnings are not regular — for example seasonal work or irregular hours — the regulation permits use of a weekly average over an appropriate period.
Regulation 38Self-employed current income
Contains the rules for determining current self-employed income by reference to relevant profits.
Regulation 39Pension contributions can matter
Under the historic-income rules, regulation 35 provides for adjustment of the HMRC figure where qualifying relievable pension contributions have not already been deducted and the required information is provided.
Regulation 38 also addresses pension deductions when calculating certain current employment income.
Historic income & pensions Current employment incomeShared care and overnight stays
CMS must determine the relevant number of nights
Regulation 46 deals with determining the number of nights that count for a shared-care reduction.
The normal assessment considers the number of nights the child is expected to spend in the paying parent's care over the relevant period.
Evidence may include an agreement, court order and other evidence showing the actual or expected pattern of overnight care.
Regulation 46 Regulation 47Special-expense variations
Some expenses can potentially reduce the income used for child maintenance where the statutory variation requirements are met. These are not general living expenses: they must fall within an authorised variation ground.
Contact costs
Certain reasonable costs incurred in maintaining contact with a qualifying child can constitute special expenses.
This can include qualifying travel and fuel costs, subject to the full statutory conditions and thresholds.
Regulation 63Other special expenses
The following regulations contain additional rules about the categories and operation of special-expense variations.
Special-expense rulesUnearned income — what the law actually says
Unearned income variation
Regulation 69 is extremely important because it defines the type of unearned income that can be used for this particular variation.
The regulation covers income of a kind chargeable to tax under specified parts of the Income Tax (Trading and Other Income) Act 2005, including:
- property income;
- savings and investment income; and
- specified miscellaneous income.
The statutory threshold in regulation 69 is also relevant.
Assets and diversion of income
The variation legislation also contains provisions dealing with circumstances beyond ordinary PAYE earnings.
Assets
Current legislation includes an assets variation provision. This area has also generated Upper Tribunal case law, so decisions should not be treated as a simple assumption that ownership of an asset automatically equals ordinary earnings.
Regulation 69ADiversion of income
The regulations contain a separate variation ground dealing with diversion of income.
CMS must apply the actual statutory test rather than simply treating every financial arrangement as diverted income.
Additional-income variation rulesAnnual reviews and changes to current income
Updating income at annual review
Deals with updating the historic-income figure at the review date where new HMRC information is available.
Regulation 20Updating unearned income
Deals with updating additional unearned income where a variation under regulation 69 already affects the calculation.
Regulation 21Periodic current-income check
CMS can require evidence to validate current income in the circumstances set out in the regulation.
Regulation 22The 25% test between reviews
Where maintenance is already based on current income, a later change in that current income will generally need to satisfy the statutory 25% threshold before another supersession can be made on that basis.
Regulation 23How to investigate a potentially wrong CMS calculation
Do not challenge only the current account balance. Identify the maintenance decision that created the liability.
Different decisions can cover different periods. A later decision does not necessarily explain an earlier debt.
Record the annual and weekly figure and whether CMS described it as historic income, current income or another category.
HMRC records, P60s, payslips, self-assessment information, pension evidence and other relevant documents may be needed.
Compare CMS's method with regulations 34–39 and any other relevant provision.
Compare the nights CMS used with the evidence and regulations 46–47.
Establish whether qualifying relievable pension contributions were already deducted from the relevant income figure.
Establish whether CMS has added unearned income, assets or another variation and identify the precise statutory ground.
Do not combine ten years into one figure without first checking every decision and effective date.
Decide whether the issue is revision, supersession, variation, correction of an accidental error or tribunal appeal.
Build a decision-by-decision evidence table
This is one of the strongest ways to present a long-running CMS dispute. It prevents different years and different decisions being mixed together.
| Decision date | Effective date | CMS income | Source | Your evidence | Shared care | Law relied on | Difference |
|---|---|---|---|---|---|---|---|
| Enter date | Enter date | £ | Historic / current / variation | HMRC / payslips / etc. | Number of nights | Reg. 34, 35, 46 etc. | £ |
| Enter date | Enter date | £ | Historic / current / variation | HMRC / payslips / etc. | Number of nights | Relevant provision | £ |
What to ask CMS for
- The date and effective date of every maintenance calculation affecting the disputed period.
- The gross annual and weekly income used for each decision.
- Whether each income figure was historic income or current income.
- The tax year used for each historic-income decision.
- The HMRC figure supplied for each decision or annual review.
- Details of any pension deductions applied.
- Details of each variation and the statutory ground relied upon.
- The amount of any unearned income included and the source/category relied upon.
- The number of shared-care nights used for every relevant period.
- The effective date of every change of circumstances or supersession.
- A full breakdown of maintenance liability separately from collection fees and enforcement charges.
- A payment history showing payments received and allocated.
- The calculation showing exactly how any present arrears balance was produced.
How to frame a correction request
“CMS has got my case wrong.”
A stronger structure is:
“I dispute the maintenance calculation dated [DATE], effective from [DATE].”
2. Identify what CMS used
“CMS used gross annual income of £[X].”
3. Identify your evidence
“The HMRC/payslip evidence for the relevant period shows £[Y].”
4. Identify the law
“I ask CMS to reconsider how regulations 34–38 of the Child Support Maintenance Calculation Regulations 2012 were applied.”
5. Identify the remedy
“Please determine whether the decision should be revised under section 16 of the Child Support Act 1991 and provide a complete calculation and reasons.”
Mandatory reconsideration
Where you believe an appealable CMS decision is wrong, mandatory reconsideration is normally the first formal challenge before an appeal to the First-tier Tribunal.
Your request should identify each disputed issue rather than simply stating that you disagree with the total.
Income
State the CMS figure, your evidence and why you say the applicable income regulation was applied incorrectly.
Shared care
State the number of nights CMS used, the number you say is correct and provide supporting evidence.
Variation
Identify the precise variation ground and explain why the statutory conditions are or are not satisfied.
If CMS does not correct the decision
Challenge the relevant decision within the applicable time limit wherever possible.
Give CMS the documents and calculations supporting each ground.
Keep it. It is an important document for any subsequent appeal.
The tribunal is independent of CMS and can determine appealable issues within its jurisdiction.
Poor service, delay or failure to answer correspondence may be complaint issues. Whether the legal calculation itself is right is generally a decision/appeal issue.
Revision or supersession? Why the difference matters
| Situation | Possible route | Key law |
|---|---|---|
| CMS made an original decision using information that you say should have resulted in a different decision | Revision / mandatory reconsideration | Section 16 |
| Circumstances changed after the original decision | Supersession | Section 17 |
| Current income changed sufficiently | Potential supersession | Regs. 23 and 34 |
| Additional taxable unearned income needs considering | Variation | 28A–28G / Reg. 69 |
| Qualifying contact expenses need considering | Variation | Schedule 4B / Reg. 63 |
| Typographical or accidental error in a decision | Error correction may apply | Section 28ZD |
| CMS upholds an appealable calculation following reconsideration | First-tier Tribunal | Section 20 |
Do not confuse these three things
Liability
The amount of child maintenance legally calculated as due.
Payments
Money actually paid and how those payments have been allocated to the account.
Arrears
The difference said to remain outstanding after liability, payments and relevant adjustments are accounted for.
Collection and enforcement law
Collection and enforcement powers also come from legislation. Sections 29 onwards of the Child Support Act 1991 contain important collection and enforcement powers.
Child Support Act 1991
Includes statutory powers relating to collection, deductions and enforcement.
Collection & enforcementCollection & Enforcement Regulations 1992
These contain detailed rules covering matters including deduction from earnings arrangements and their operation.
Read the RegulationsNewer enforcement legislation
Child Support (Enforcement) Act 2023
This Act amended the statutory enforcement framework, including provisions concerning liability orders and administrative enforcement.
Enforcement Act 2023Child Support Collection (Domestic Abuse) Act 2023
This Act changed the law concerning collection arrangements in cases involving domestic abuse.
Domestic Abuse Act 2023Essential official legislation links
| Law | What it is useful for | Official source |
|---|---|---|
| Child Support Act 1991 | Main statutory framework for calculations, decisions, variations, appeals and enforcement. | Legislation.gov.uk |
| Schedule 1, Child Support Act 1991 | Basic maintenance-calculation structure and shared-care reduction framework. | Schedule 1 |
| Schedule 4B | Grounds for variation. | Schedule 4B |
| Child Support Maintenance Calculation Regulations 2012 | Detailed current-scheme calculation, income, shared-care and variation rules. | SI 2012/2677 |
| Social Security and Child Support (Decisions and Appeals) Regulations 1999 | Detailed rules concerning revision, supersession and decision-making procedures. | SI 1999/991 |
| Child Support (Collection and Enforcement) Regulations 1992 | Collection and enforcement procedures including deduction from earnings. | SI 1992/1989 |
The strongest way to challenge a CMS case
A strong challenge does not simply produce hundreds of pages of correspondence.
For every disputed period establish:
- what CMS decided;
- when the decision took effect;
- what income CMS used;
- where that income came from;
- what HMRC or other evidence actually says;
- what pension deductions were relevant;
- what shared-care nights were relevant;
- whether any variation was applied;
- which legal provision governs the disputed issue;
- what the corrected calculation should be;
- what payments were actually made; and
- what arrears remain after correction.
That turns a general complaint into a decision-by-decision evidential challenge.
One particularly important question
If CMS says income was £20,000, shared care was 52 nights, an additional £5,000 constituted unearned income, or a particular change could not be backdated, ask them to identify:
- the decision;
- the evidence relied upon;
- the relevant regulation or statutory provision;
- the calculation performed;
- the effective-date rule used; and
- the reasons for rejecting contrary evidence.
This makes it much easier to establish whether the disagreement is about evidence, interpretation of the law, calculation, or administration.
Right First Time principle
A parent should be able to see what income was used, why it was used, what adjustments were made, which shared-care figure was applied, what variation was included and how the final liability was calculated.
Where that chain cannot be established, the answer should not be simply to accept the account balance. The underlying decisions and evidence should be checked.