Hi and welcome. Am sure one of the experts will be along soon but basically you pay into an IVA and disposable income so if you earn more money from your job and your out goings remain the same the IVA would take the extra. If your having a baby and your expenses go up but your income is to remain the same or less you would need to speak to your IVA provider and explain the situation. The best rule of thumb is if anything changes for the better or worse let your IVA provider know but unfortunately if you do have a promotion you will not see the benefit of this until the IVA is done.
If you earn more money, then your payments are likely to go up unless you can offset this against higher expenditure.
If you have additional expenditure then you need to speak to your IP about it. You might be able to get a reduction in payments or a payment break.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
You will generally never be asked to contribute more than 50% of your increased income - and this is after all rises in expenditure have also been taken into account.