As a Sonata owner, it is your responsibility to ensure that your boat complies with the class rules. If it does not comply fully, then it cannot race as a Sonata! Even if you do not intend to race, following the rules will contribute to the boat’s safety and it will help maintain the value of your boat if you can say with confidence that it complies with the rules. The latest edition of the National Sonata Class Rules is available free as an Adobe Acrobat .pdf file from the RYA Web Site.
To prove that your boat complies with class rules you’ll need a measurement certificate. When you buy a second-hand Sonata make sure that it comes with a certificate. Read more on certificates in How to Get Your Sonata Measured by Mike Owers.