Step parent here. SD 19, DS 17 and 14. We live in a 3 bed house. Boys used to share and SD had own room. She used to come to use one weeknight and one or two weekend nights every week pre Covid. Post Covid she was older, wanted to spend her weekends with friends, didn't need picking up from school midweek and visits became more infrequent. No falling out, just very typical teenage lifestyle change. She spoke with me a couple of years ago and suggested that her brother have her room as he's getting older and needs his own space. I said "but what about you, we want you to feel welcome and have your own space" and she said "I don't sleep here all the time, when I do he can sleep in with (youngest) on the bunks" so we compromised, it's become the 17's room but her stuff is still in there as well as his. She very very very rarely sleeps here now. She works full time including weekend shifts. We do see her and our relationships are all good. We could only afford a 3 bed in the cheapest town 15 years ago and that's still pretty much the case! Before that we were in a 2 bed flat where the 2 older ones shared a room. I say all this to give you a reason to have a conversation with the 19 year olds especially. "we're looking at moving to another house and just want to chat about what you'd want in that house" If the 19 year olds could share dorm room style, that would at least make some difference. 5 beds are so expensive. Are the 19s staying with you for the full weekends every weekend? And they live an hour away? What do they do when there are parties? I see they're still in 6th form college but do they have part time jobs? Sports? Hobbies? It's hard to see the teenagers who live with me full time because they're so busy!
Are there any housing options with a granny annexe type set up or a home office shed thing or garage that could be converted into a bedroom? Or a reception room that could double up as a bedroom and something else?
I totally understand the need to make the SC feel welcome and that this is still one of their family homes.
However - and this is a big point - if your husband is the one who's absolutely immovable about the number of bedrooms, could he not just earn more?! And make the difference that would mean you could afford a bigger place in the area you want to be in?!
Finally, if your youngest is bright and doing well, if they've got good support at home and access to all sorts of opportunities and conversations, even if they go to the closest school, they can still do very well. My boys both have been at our most local high school and it's been in special measures, all sorts of things, and they've both been top 5-10% in all subjects, the older one did well in GCSEs, has good predicted A Levels and offers from every uni he's applied to, including Russel Group red bricks. Sure, they've had to be a bit more resilient but home is where the biggest difference is made. The more "successful" school in a posher local area that we can't afford to live in should also have to share the data on how many of their pupils attend paid for tutoring because every teen I know at that school is going to tutors on top of their time in school and everyone sings the praises of how wonderful the teaching is there. If it's so wonderful, why do you all need these extra classes?
Anyway. Talk to the SC about the home situation. They'll probably take for granted that you'll always live in the same place. Talk with your husband, share this whole thread with him.