If your application is refused this doesn’t mean your project won’t happen at all and it need not incur huge fees.
To start with, you may have advance warning from your planning officer that your proposals are facing rejection so might be able to modify certain aspects so it can be approved.
If permission is denied outright, you can appeal against the decision and you might decide to employ a planning consultant to manage your appeal (make sure they're a chartered member of the Royal Town Planning Institute), if this is the route you want to take.
You can also re-apply, without paying another fee to the council. Generally the council offer a free re-submission service, as long as this is within 12 months of the decision of your first application.
The redesign should of course take into consideration the reasons for refusal and any advice you have been given. You could even modify your design so that it falls under permitted development.
Don’t forget that building regulations approval will be needed for any extension or conversion whether planning permission is required or not.