I turned my 4 bedroom terrace into a 3 bedroom terrace, and what was the second-largest bedroom is now a bathroom with underfloor heating, a massive rain shower, a free-standing tub in which I can soak up to my neck, and a teak bench to lay about on for a bit while I dry off. It's full of tropical house plants and I basically live in it. It cost 25k and now the house is worth about 30k less than it was before. Everyone thought I was an idiot but I fucking love it and I'd do it again in a heartbeat.
The original bathroom is still there, I plan to turn it into a laundry room but haven't gotten around to it yet.