I'm looking at getting an SSL certificate and I think the SAN type is what I need. However, Can I easily add further domain names to be covered by the certficate in the future. E.g. I have www.example.com on my server. In 6 months time I decide to build and host a new site on the same server and give it the URL www.example2.com. Would I then be able to add the new domain to the existing certificate?
Certificates cannot be changed after they are signed, otherwise they would provide no security.
Once issued, a certificate holds all SANs. This means that a certificate would have to be revoked and a new one has to be issued to add a new SAN.
You should first find out which names you want to register, because revoking and reissueing will most likely cost extra money.
Its not possible to add domain in the existing SAN SSL Certificate, you can temporarily self-signed the certificate for the other domain you are thinking for and then at the time of expiration(after 1 or 2 year validity) you have to apply a new SAN to include your new domain name.