Lunatic designs and the law...

Hello all. First post here.

What does a plain standard and normal registered boat, if modified to
very unusual characteristics has to comply with in order to legally run
?
Does it have to be presented to some authority for evaluation ?
If planning to home build or modify, would presenting its plans in
advance be a better choice than spending the effort to risk a later
negation ? Or plans is not enough, but engineering calculations be
needed ?
Thanks,
Miguel