Okay - obviously too many messages to address everything, so sorry if you’ve provided constructive advice and it looks like I’ve ignored it - I have just sat for the best part of half an hour reading all your responses. Thank you for all the advice!
To address some points - the attic is split in two rooms, one is a much bigger room (where DD1 and DD2 sleep), and one an average sized bedroom (where DD3 and DD4 sleep). Currently, I would probably move DD3, DD4, and DD5 into the bigger room and getting DD1 and DD2 loft beds with desks under. I’m not worried about them needing study space because they actually study together a lot (only one school year apart) - not necessarily doing the same things, but just sitting quietly and working at the same time and occasionally asking one another to quiz each other when revising for tests etc. I don’t see this changing as they approach GCSEs/A levels. They shared a room before they had to just because they wanted to. Our bedroom is a bit smaller than DD4 and 5’s room - we sacrificed space for an extra bathroom!
The problem with DS’s box room is that we’ve built a loft bed into it (literally attached to the wall) so that he can fit a desk and dresser in there, so it’s not really viable for the baby to go in there and switch it about. That’s why I was thinking in three/four years, when DD3 is 9/10, it would be good for her. And DS does still use the room, the long summer is enough to justify him keeping it for now really! And I’m not expecting him to live at home for more than a year after he graduates, no. Maybe that’s unrealistic but I’m hopeful that it’s a strong course he’s doing and he knows where he wants to go with it and the paths from there are direct. He’s already saved a lot of money from the part time job he’s had since he was 16 - he’s built up to be a manager on duty four shifts a week, so i think he’s financially set to move out.
And when the younger girls are 14, 11, and 9, this house will be more than big enough for them to have their own spaces/rooms because DS and DDs will be 27, 22, and 21. This “squash” as it were would only be temporary.
I’n reluctant to move because the house itself is lovely and we’re on an amazing street very near both (good) schools and near all our friends and our church. It’s just the perfect house for our family at the moment, but I am willing to move if that changes with the arrival of the new baby. I’m just thinking about options so we’re prepared.
The graden suggestion is interesting - we already have an outhouse like the one I think you’re all describing. It’s used as a music room/dance practice space/slump in bean bags space. We built this about three years ago because our lounge/kitchen/dining area is all semi-open plan. Extending the house isn’t really an option without losing all our garden space, we’ve already extended the kitchen as far as we’d like. We could potentially build something above that but I like the sky windows and all the light. Makes mornings much easier!
And to someone who got confused - I’m expecting DC6, who is DD5. I have 1 son and (soon) 5 daughters.
Thank you to all of those who didn’t judge!