What is the most isolated building in the world?

Answers to this type of question generally hinge upon exact meaning or interpretation of words. In this case "isolated", "building" and "in the world" seem to be key.

Since the concepts of isolation and what constitutes a building have already been discussed it seems relevant to address what "in the world" means. Taken literally it seems to imply within the planet earth, or under the surface. However, I suspect Glorfindel had a broader meaning in mind. Therefore, considering from a wider perspective, I'll expand my answer to include our entire solar system. With that in mind, the most remote structure which has helped house humans and remains intact to this day would be one of the NASA Apollo mission lunar landers.

There are 6 Apollo landing sites. Based on their distribution it appears that the Apollo 15 mission was the most remote. At each site the lower half of the lunar module along with many scientific instruments and whatever they no longer needed was left behind. This reduction of weight allowed the astronauts to maximize the amount of lunar samples they brought back. At the Apollo 15 landing site an electric lunar roving vehicle was also abandoned.

It may well be the Earhart Lighthouse on Howland Island, smack dab in the middle of the Pacific about 3000 km from Hawaii:

(Courtesy Joann94024, Wikipedia)

https://en.wikipedia.org/wiki/Howland_Island

https://maps.app.goo.gl/wHMGRizq7uqzYeaD9

Built in 1937 for Amelia Earhart's attempt to cross the Pacific and promptly abandoned. It's not entirely clear if it was ever really a habitable lighthouse, but it certainly is a "structure for human activities", and on casual inspection I can't find any other structures for hundreds of miles around. (Baker Island is not far away, but it appears to contain nothing at all.)

IMPORTANT NOTE: From comments we have a photographic proof that in recent years there were two buildings (but in the same platform), so this answer do not fulfil the question requirements (later precised: Arctic research stations count as multiple buildings [which are similar to this]). (Until nature will destroy this station, and maybe a new one-single building will be constructed).

I suspect sometime the most isolated building is a hut/radio station on Bouvet_Island. Note: In theory, they also have an internet top domain (.bv).

Looking from Wikipedia and maps, I'm not sure there is only one single building, but it seems so. Note (from Wikipedia), sometime buildings were destroyed, and years later rebuilt. They may be inhabited for few months from time to time. So I assume, when the building is not destroyed by nature, it is the most remote building.

Why do I suggest the hut/radio station on this island? This island is the most remote place on Earth, so there is a good possibility that it also has the most remote buildings (also because I found no references that Bouvet ever had 2 or more huts).