It may vary from area to area but the only way to find out is check. I am
almost certain that if you have a dwelling that is more than 2 storeys high
with living accomodation then a fire proofed escape way has to be provided.
This obviously affects most people that do loft conversions except those
with bungalows. You already having 3 floors may already know this but of
course if your house was built/converted prior to these regulations it may
not have affected you. If you go ahead yo may be required to also undertake
work to your existing property to comply with all fire regulations