Anything affecting your home's structure — removing walls, cutting joists, new openings — needs Building Regs approval.
Part A requires that buildings remain structurally safe. It applies when you alter anything load-bearing: knocking through walls, enlarging openings, cutting into joists or rafters, or adding significant weight like a loft conversion.
A non-load-bearing stud partition wall generally doesn't need approval — but check what's above and below before assuming. If in doubt, a structural engineer's calculation is cheap insurance.
For notifiable structural work you submit either a Building Notice or Full Plans application to Building Control, and an inspector signs off the work in stages.
This is guidance, not legal advice — always check with your local Building Control before starting work.