There is no such thing as an actual legitimate guildhall in ESO. There are player homes that they allow a guild to use. This is something I think Zos needs to focus on as it is the most important feature of housing they have left out of the picture.
Edit; If Zos does every allow a home to be an official guildhall, one that is actually owned by the guild, it will likely require a fresh purchase be gifted to the guild.