Before adding that bathroom it might be worth a call to your local planning department!
A 'change of use' permission may be required especially as you've also increased the roof height.
Getting a stop notice is no fun, especially as you don't plan on renting it out to students.
Building control might want to look at the foundations (or lack of) under that new wall you built too.
But then again I'm sure you've already done that, just didn't want to bore us with legalities