When we bought our house, the existing bathroom was no more than 2x1meter, completely worn out, humid and moldy. Dreadful. Not much in there we could use again, so we decided to give up the third bedroom and create a space to unwind and get clean. The old bathroom would do much better as a small walk-in closet.
So we closed up old bathroom door with a full height cabinet and drilled a new entry towards the bedroom. Wainscotted the walls with plywood and built in as many shelves as possible. I found this sweet black wallpaper to match the plywood and chiseled off all the tile glue off the original terrazzo floor. I have no full after picures, because the place is packed (we're still busy renovating our house).
The bed-now-bathroom is 3x2meter, a nice space to put in a bath, sink and shower. We added the former hall-closet and took out the unused chimney, so we could also fit in a toilet. The original pine floor was not strong enough for the weight of the bathroom, so we built a reinforced conrete floor from scratch with heating in it. Put in all the plumbing, rewired the entire room (we can now choose from three types of light, for every occasion). Built a wall with glassblocks in them to have some daylight in the shower. Drywalled the ceiling (there was softboard throughout the house) and tiled the floors and walls. Chopped up an old oak tabletop, sanded it down and turned it into a washstand. Added a few other features and fixtures and a huge mosaic on one wall.
I must admit I am a little proud of us and very thankful for our skills, because we do it all with our own hands.