I don't think your cheap extension idea is legal or safe tbh.
But sleeping on a normal sofa isn't suitable either.
Can the living room be designed to be flexible so that it's a bedroom at night and a living room in the daytime?
For starters get a good quality, sprung metal action sofa bed, with a decent mattress and proper bedding for it. Or perhaps even a Murphy bed?
Perhaps consider a room divider or even something like a pony wall?
Have a blanket box cum coffee table for stashing the bedding during the day.
https://www.ikea.com/gb/en/ideas/the-24-7-living-room-pubbdec7210
https://designingidea.com/space-saving-beds/
Scroll down to "transforming beds"
The relationship is a separate issue of course. Have you told him how unhappy you are and that as far as you're concerned the relationship is essentially over?
Have you even looked into possible solutions to having enough income, support with ds etc if you were to properly split? (Or indeed if dh died) I'm thinking if it was 50/50 residency for ds then you'd at least get 3? Nights of total respite a week? Surely that would free you some time to make friends etc too?
Split the largest bedroom into 2 you and DH get a single bed each.
The dh snores so without a proper wall which I think would require planning permission health & safety assessment etc I don't think that would work.
@Bluesandwhites suggestion of "nesting" could work too?
I’d say so . We never eat together . We never sit in the same room...
Sorry but that rather sounds like you haven't actually been clear with him and whatever his faults that's cruel. You need to tell him as he has a right to decide whether to continue living with you if you are no longer in a relationship with him.
Do you not have a family social or support worker?