It appears that Brussels' World Heritage Sites are increasingly getting de-Stoclet-ified! Having noticed that no-one appears to have reviewed the Major Town Houses since the Hotel van Eetvelde was opened to the public, it seems pertinent to share my experience.
I managed to fit three of Brussels' four World Heritage sites (and two of its tentative sites) into a six-hour layover. Had my inbound flight not been delayed, I would have made an effort to cram in the Sonian Forest(s) too, but there are no shortage of beech forests for me to make a pilgrimage to at some point in the future.
In 2023, set to coincide with the 130th anniversary of Art Nouveau in Brussels, the Brussels government funded LAB-AN to be a permanent centre for the promotion of the city's Art Nouveau heritage, located in the former rental annex directly next to the Hotel van Eetvelde.
They also engaged in legal battles with the private owners of Horta Houses and Stoclet House to make them open to the public. The Hotel van Eetvelde is now open three days a week and booking is available online. There is the option of a guided tour, but I asked the staff there and they said that many days it does not run from lack of interest. LAB-AN has several permanent exhibitions about the Hotel van Eetvelde that you walk through before gaining access to the building itself, so I felt it was well illuminated. I booked a couple hours before arriving and there were no booked-out time slots, so it was definitely something that could be done spontaneously. I got the one remaining time slot for the Maison/Atelier Horta on the same day (which I think has been adequately reviewed!) and the Hotel Solvay appears to be open at only random days every month, so I did not have the opportunity to go inside. The Hotel Tassel is the only one not regularly open to the public, so another I had to satisfy myself with only seeing from the outside.
A hilarious result of the legal trouble is that LAB-AN now also has a room dedicated to a "permanent exhibition" on Stoclet House. It consists of a 40-minute long film painstakingly showing all of the interior of Stoclet House. I think I was the only one to watch the whole thing and certainly the only one there to understand the hilarity of the thing. I considered sitting through this dedicated museum exhibition on the world's most infamous World Heritage Site (located in another WHS that was until very recently closed off for very similar reasons!) a prerequisite to 'ticking off' Stoclet House later in the day.