Kind of depends on where you are. In the USA, if there are
no state or local laws forbidding it and it doesn't fall under
the arms controlled by the 1934 NFA, go for it. Even some
of the NFA items can be "homebuilt"---SBR, suppressor,
SBS, maybe even AOW. You just have to play by the rules and
have your paperwork in order BEFORE you build.
Want to design/built full auto? That takes a license. Want
to build guns for profit? License. For your own use or to be
the mad inventor toiling in his garage on conventional
firearms? Go for it as far as I know.
Keep in mind that the above statement is probably legally
worth exactly what you paid for it.