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I’m pretty sure the existing structure isn’t non-complying
Well, maybe, maybe not. Your Building Control cert (or the lack of one) will tell you.
But that's not the point. Point is, if you ask someone to build a conservatory, it has to comply with the regs for conservatories. Nobody will build you an extension at conservatory prices that looks like a conservatory and get you a BC cert for a legit extension.
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.