There is an old saying among yacht brokers. If you build a "one off," then when you want to sell it, take half off. That applies to a one of a kind custom build from an established builder, selling a back yard build could make your project very expensive.
The "legal reasons" are less important than insurance reasons for most owners. Insurance companies will have a multitude of requirements to insure a back yard build.
Tucker Fallon CPYB