Legal regulations for eBikes

Hey everyone!

Been thinking a lot about DIY eBike builds lately, and it got me wondering about the legal side of things. If I were to build a custom eBike from scratch, say a Class 3-equivalent with a powerful motor for some serious hill climbing, what are the main regulations I need to be aware of? Are there specific power limits or speed cut-offs that are different for DIY builds compared to factory bikes? Don't want to accidentally build something that's not street legal!