Changes to leasehold flat

[Twoshoesnewshoes]

Could one, in theory, make significant changes to a flat which is leasehold? Eg swap kitchen with bathroom?

yes but you might need permission from the leaseholder.

Depends on the conditions of the lease. I'm pretty sure I'm allowed to decorate how I want internally but not so anything structural or external.

Thank you, that’s helpful

Does your building have a managing agent? If so, they will advise what consent you need and how to get it. Check your lease as well.