If your loft conversion is habitable, it probably needs building regulations.
A Velux window is usually fine but a dormer window (which sticks out) needs Building Regulations Consent.
Fitting fixed stairs, rather than a pull down loft ladder, requires Building Regulations Consent
Installing electrics or plumbing requires Building Regulations Consent.
Cladding the walls (putting up partition boards) so you don't see the timbers again means you need to consider planning and building regs.
Proceeding without Building Regulations Consent can delay your eventual sale. Indemnity insurance or retrospective consent is going to be needed.