Fingerboard extensions are very common in the USA, but become more and more popular in Europe, too. They have a couple of advantages over a 5-string bass, can get fitted to any 4-string bass and if they are well made, it’s reversible.
My friend Giorgio Pianzola, bass maker from Bern/Switzerland, has published a comprehensive guide how to calculate the length of an extension very exactly.
Visit www.kontrabassblog.ch for further reading (it starts in German – scroll down for English translation).
I am doing a base shop in Korea.
In Korea, there is no place to work on c-extension, so I’m going to try it. There might be a misunderstanding using a translator. Please understand