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Sure, all of which can be done if you can cut straight and drill a straight hole, muddling through the various problems. Self-build in the UK is woefully under-represented compared to places in europe, and there have actually been relatively successful self-build movements in the past: https://www.scopeofwork.net/segal-method/
A glorified shed is a great place to start if you're into that kind of thing, i would have thought?
(Edit: Not at all undermining that it's still relatively complex, but it sounds pretty fun to me at least)
Of course it's part of it, but you'd still need to learn how to cut and measure beams, assemble a frame, build a durable, insulated watertight roof that won't blow off in a gale, put up stud walls, route services (OK, a GR just needs electric unless it's a palace), apply cladding, insulate it, put down a floor, but the windows in... it's endless. And it has to last decades so don't make a minor mistake that means the whole thing is fucked in seven years of UK weather ok?
No planning, well, kind of. They are generally PD but there's loads of shit you can pull to build something far larger than what PD should allow (which is fuck all) but you need the expertise to pull that off too.