There is very little that I can add to what the other reviewers said before me about this underwhelming site. The optics of bestowing a World Heritage designation on something that you can only view from a distance through the fence definitely feel wrong. It is clearly an unusual building - and all available descriptions of the interior suggest that it would be marvelous to explore - but until such time that it becomes possible, any visit would be unsatisfactory.

That being said, checking this site off your list is pretty easy if you are already in Brussels. Stoclet House is located some distance from the city center, but a taxi ride there and back will not burn a hole in you wallet. The ride did not take 15 minutes one way. I got the driver to park and wait for me while I gazed at and took a few pictures of the building through the aforementioned fence for about another 10 minutes. At the current limits of accessibility, I have no problem counting the site as visited on the strength of making an effort.