Ok Maggie. Thanks for the mention.
Since this is a public website, I'd better give it to you "by the book".
Officially, this work is "notifiable" to Local Building Control"
Part of the local Planning dept.
I would use a "Building notice" rather than having costly plans drawn up. Simple application form, then your builder liaises with the Building Inspector.
That's the official way. It is worth it because, if you sell, you'll be asked if anything's been done that should have been notified.
First thing is to get builder/plumber in to quote. To see if you still want to go ahead.
I guess you'll want a new doorway from your bedroom to the dressing room?
It all sounds pretty straightforward to me.
Electrician will be needed to wire in an extractor fan. Easier if you have an outside wall. He may need to move the light switch too.
Above existing bathroom is perfect. Location of WC may be dictated by the position of the soil stack (big poo pipe), or it may already go up through the room to the roof. Builder will advise.
That's enough for now. Discuss layout etc with whoever is quoting.
Keep us posted.