The payment in an IVA is basically what is left once you take agreed living expenses from your take home pay. If the pay increases, so does the IVA payment, usually by half the increase. Overtime and bonus payments are also due to be paid to the IVA each month on top of the regular payment, usually you are allowed to keep an amount equivalent to 10% of your agreed pay and the rest is split 50/50 with the IVA (We usually refer to this as the '10% 50/50 rule'). Additionally windfalls over £500 are paid in as well. Finally, if you are a homeowner, equity release will be looked at near the end of the IVA.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk