He could directly deny that there is a new district office, the old one only had received a major reconstruction. Though I had been on Samui once several years ago, I did not take photos of the administrative offices at that time, so I asked him to do so when he has time and is in the area. Faster than expected, he did the tour and wrote about it in his blog.
Not mentioned in his posting is the district office, about which he said it received some renovations, and also now has more parking space. If you take a close look at his photo placed in top of the posting, you'd notice that the sign actually says "Koh Samui District" - not following the RTGS transcription system usually used in the government offices. The recommended transcription in Ko Samui, but the spelling Koh for the Thai word "island" (เกาะ) is so widespread it seems the Royal Institute cannot eradicate it - same as Pattaya should actually be transcribed as Phatthaya.
And in fact there is even a third newly built administrative office, as the provincial court Ko Samui (ศาลจังหวัดเกาะสมุย)in the south of the island was opened only few years ago. I marked all three buildings in the map below, but note that the municipal building is hardly visible because it is located directly at the boundary between hires and just recently added medium resolution imagery, thus a bit blurred. And since the hires data is from 2003, there's obviously no trace of the new building in there.
View Samui administrative offices in a larger map
So, many thanks to Camille for providing me with the information as well as allowing me to use his photos for display here. Now whenever the special administrative area of Ko Samui gets closer to be set up I have at least a photo of the municipal office to decorate the posting.