One of the things we stumbled upon at the Marché aux puces de Saint-Ouen (besides some mid-century travel brochures) was a Futuro House. I took French in college, which means I don't speak a word of it anymore, so I assumed the sign that said Futuro House just meant house of the future, which looking at it, makes sense.
And the velvet rope just made me assume it was some kind of one-of-a-kind art piece that you weren't supposed to touch or enter. However it turns out that the Futuro House is a real space age house, with its own Wikipedia page. This is the only one in France (maybe, my broken French really wasn't helping with that sign). The part that I could understand is that the Futuro House is made of fiberglass, was designed by a Finnish architect, Matti Suuronen in 1968 and that only 100 were manufactured.
You couldn't go inside, but you could peak your head (and camera) in to see all the space age furniture and lighting.