[Innomare]

Hi DavidG,

if the yacht is truly a private yacht, and not intended to be chartered (commercial use), it is not mandatory to comply with MCA or SOLAS regulations (as far as I know). It may make a huge difference in insurance payments though.

Most large yachts (even private ones), are built to comply with the regulations for charter yachts (MCA Large Yacht Code e.g.). If not for anything else, at least for a possible future resale, or for the increased safety level.

In the MCA code there is quite a big difference between sub-500 GT and over 500 GT. Mainly regarding structural fire protection (use of combustible materials), mandatory sprinklers etc. Also ISPS becomes mandatory (security regulations).
Nonetheless, it is perfectly doable to build a yacht over 500 GT and yards are building them all the time (Lurssen, Feadship, Amels, Oceanco, ...)

It's a lot worse to go over the 12-passenger treshold than over the 500 GT, because over 12 passengers, you exit MCA-code and enter SOLAS grounds (small passenger ship).
At 55 m, it'll be tough to stay under 500 GT, unless the design is very sleek (narrrow beam and low profile). Also keep in mind that many boats grow a few metres during the design process.

Good luck with your project,

Bruno