Maintenance - is rent taken into account

[MrsS01]

My ex is currently renting a property (he can't afford to buy) and paying maintenance for my kids (more than CSA say). When working out maintenance are his outgoings - such as rent/bills taken into account before maintenance is worked out?

No, never.

You say he's paying more than he should but (without being rude) you clearly have no grasp on how CSA works.
Go on the website and do the calculations yourself to find out the score. Google: CSA Calculator.

The calculation is done on 'net' wage i.e. wage he receives after paying tax and NI and anything else i.e pension/student loan payment etc that goes directly from his wage packet before it goes in to his bank account.