Originally Posted by MackemSi
£150 for the application.
It will cost you for the building regs as well. That is more expensive and not necessarily a set cost.
Building regs approval is often based on the value of works to which the building act is applicable. Assumption based on info provided, that would be pretty much everything.
If you're unsure you will get planning permission, best advice is to speak with the local planning authority before. Have an informal chat to see what your chances are (you won't get a decision and they won't be held to anything said) but this should give a reasonable indication of how successful your application will be without the initial outlay.
Bear in mind that you're job may be more expensive that at first glance, in particular when trying to satisfy building regs. Most notably fire safety. You're building over a garage so risk of fire is a big factor. Pay particular attention to approved document 'b' when designing.