Guidance for all changes within child maintenance and child support schemes.
Is it Income?
Any expense that is shown to be "wholly and necessarily incurred" for the purpose of your employment should be disregarded as income. A classic example is a car allowance, which was deemed to be excluded as income - or at least, any proportion of the allowance that is related to business use. The CSA may therefore require clarification of how much of your car is used for personal use compared with business use.
The Commissioner ruling over a car allowance can be downloaded here