When I moved (downsized) to my last house it was a terraced 1930 house. At that time, only one of my adult children had a child.
Within a very short time, other g.children started to be born, and I decided to have just one big room put in the loft for them to sleep (and when older,_ to play in.
Used a local company and decided to keep down costs and times, just to have that one room, with stairs running up from landing where there had been an airing cupboard which had originally houses a large bot water tank. That I replaced with a modern combi-boiler in the kitchen.
Kept the original door to airing cupbaord. As those stairs led directly into that room, it meant that the the loft only every (legally) counted as a 'boarded out loft', not a room. This meant it did not effect my value for Community Charge, and I did not need to have all room doors in the rest of the house changed over to fire resistant ones. Just one of those legal technicalities, Had two large Velux windows there These did need to proper Velux blackout blinds.. Lots of room, I had two single beds, masses of space for toys, rail dressing up clothes, cupboards, and had a tv airial point in there also, etc. etc. Each side had excellent (lockable) storage space under the eaves.
It would be very simple for any future owners of the house to add in a wet room or shower room with a small extension directly over the bathroom on the first floor. Of course if this was done it would then become a loft extension and the other legalities would need to take place.
I used it as a spare guest room for many years whilst I was there. I replaced what had been the little cupboard that had been at the top of the original airing cupboard with a pane of glass (so this was at the bottom of those stairs directly on the first floor landing.. As my g,children grew older, and stayed with me overnight, they could never quite understand the magic which meant that Grandma, ALWAYS KNEW when they did not put the lights out in that room when they were told to do so at night!!!!
So, I was very pleased with that and I am sure it was an added selling point last year. Gave so much extra space.