Just seeing the recent discussions on visiting the Van Nelle Factory I thought it may be useful to keep tabs on various Open House days around the world that may open up some bits of WHS that are otherwise out of bounds.

London: Usually third weekend in September (for 2015 it is the 19th & 20th). Bits of all four of Londons WHS are usually involved, which can provide something of a saving for tourists. However these are normally ones that you have to book in advance for worth keeping an eye on the website.

England: The weekend before London (for 2015 10th - 13th September). I don't know too much about how many WHS get involved in this one, but again the website may reveal some details closer to the date.

Brussels: The Art Nouveau Biennial happens in odd numbered years, over the course of weekends throughout the year, several of the Horta Houses are involved, they now seem to involve a large entrance fee €40.

Netherlands Open Monuments day seems to be in October as well, with the Van Nelle factory being open.