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I'm sure you've got a cousin you could ask...
Depending on what/where it is, you don't necessarily need to design one from scratch - a suitable product (custom-made) will come with the structural calcs for the balustrade part, the fabricator may be ok with calculating for the fixings or that might need an engineer.
Protection from falling - height and resistance to loading (and design so your child can't easily climb it or get stuck) - comes under Part K of the Building Regs. I assume if you're replacing one you still have to get building control, but actually I don't know.
Good news is that the whole roof structure and covering is coming off so it can be done properly. Bad news is I don’t know what done properly looks like.
(Also appreciate this isn’t extension but it’s not DIY either or owning your own home fare)
@hoefla halp!