15/09/2026
This is not a new income threshold for who has to pay child maintenance. It is a change to how much your income has to rise or fall before CMS will recalculate your maintenance between annual reviews.
At the moment, CMS normally uses income supplied by HMRC, usually based on the latest available tax-year information. If your current income changes, CMS generally will not replace that historic figure unless the difference is at least 25%.
The Government wants to reduce that 25% tolerance to 15%.
So the important thing is that it works in both directions.
If someone’s income falls substantially, they would potentially be able to get their maintenance reduced sooner. If their income increases substantially, the receiving parent could potentially get the maintenance increased sooner.
This is particularly important for people with commission or variable earnings
This is where the reform could become quite relevant.
Imagine someone’s CMS assessment is based on a tax year where they happened to have a couple of unusually good commission months. CMS might calculate their liability using, say, £48,000, even though their more representative current annual income is £39,000.
Under today’s 25% rule, that is not normally enough to get CMS to switch to current income. The person can therefore end up paying maintenance based on an income substantially higher than they’re actually earning.
Under the proposed 15% rule, an 18.75% difference would cross the tolerance and CMS could potentially base the assessment on the lower current income instead.
Conversely, someone assessed at £39,000 whose current earnings rise sufficiently could have their assessment increased sooner.
That’s essentially what Andrew Western means by making assessments respond more quickly to changes in people’s financial circumstances.
There’s another significant proposed change
The Government also intends to bring unearned income into the normal CMS calculation.
That can include things such as:
* rental/property income;
* dividends;
* interest from savings or investments.
Currently, these types of income can sometimes be brought into a CMS assessment through the variation process. For example, current rules can allow a variation for certain unearned income of at least £2,500 a year.
The proposed system would bring unearned income into the standard calculation, rather than necessarily requiring the other parent to identify it and request a variation. The exact mechanics still have to be set out in legislation.
The really important caveat
Nothing has changed to the 25% rule yet.
On 8 September 2026, the Government specifically confirmed that changing the threshold from 25% to 15% requires legislation and Parliamentary approval, and that an implementation plan is still being developed.
So if somebody contacted CMS today and said:
“My income has fallen by 18%, therefore you have to recalculate it because the threshold is now 15%.”
CMS could correctly say no — the existing 25% rule still applies.
The 15% figure is currently a Government commitment/proposal, not yet the operative CMS rule.
And one other thing: the normal annual review isn’t disappearing. CMS will still review calculations annually; the 15% proposal primarily makes it easier for significant income changes to be recognised between those annual reviews.
Given the issue you’ve previously described with CMS using unusually high commission months to assess income, this particular reform could be quite significant in cases involving fluctuating earnings, because a 15% tolerance is much easier to cross than 25%.