In a recent post on tiny Polish houses on rails, I complained about the use of expandable pop-out units: " Pop-outs like this are common on recreational vehicles, but I am not sure it makes any sense here; transportation to the site is a bit more economical but the added complexity and cost of construction will probably be much higher."
Where they do make sense is on vehicles that are on the road, like campers and RVs that are subject to legal limitations on width or other considerations, like with this Volkswagen Transporter conversion, the Danbury DoubleBack, where an insert pulls out and almost doubles the interior space of the van.
It's almost TARDIS-like, there is so much room inside what otherwise is a typical van. When it is sealed up it drives like a conventional van, but opens up to have the space of a real mobile home.
Then there is this Australian Gidget teardrop trailer that pops out to almost double its size.
Teardrops are light and aerodynamic, easy to tow behind small cars, but they don't have a lot of space. The Gidget cleverly pulls apart and becomes much bigger. It even exposes solar panels that charge its batteries. Living in a shoebox has a gif of it opening and closing:
Both of these examples add complexity, weight and weatherproofing challenges to what were pretty basic designs. However they do provide a lot more space in a very short time. It's a concept that might be applied to residential design, perhaps pop-out rooms when you need them that fold back when you don't.