Small update: It has turned out that an extension is not possible after all, as the existing building would not have "survived" it... Now we have been verbally assured that the building will be demolished and rebuilt, including the approved extension. We are currently waiting for the written permission, which is still pending. Since this is a [landscape protection area], our architect always tried to get the responsible environmental authority on board first and repeatedly approached the building authority, and that's how it came about. So far, this has cost a lot of nerves, time, and a nice amount of money in the lower five-figure range (lawyer fees, architect fees, structural engineer report, surveying engineering office, construction company....) without having received a valid building permit to date. So anyone considering undertaking something like this... can expect quite a lot.