what is the question ? I think you will need planning permission.
I am almost 100% sure it needs planning.
The question is whether it’s a useful halfway house and, if so, why you don’t see it done more often.
For context, initial quotes from D&B companies to do a fairly basic spec side return are north of £150k at current materials and labour prices.
why you don’t see it done more often.
Probably because you aren't really allowed to do it - unless it complies with the regs around conservatories, i.e. run on a separate heating system, have doors separating them from the other areas of the house etc.
@NickCJ started
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what is the question ?
I think you will need planning permission.