Ummm, either you mixed units or your parking spots are very large.
15m² is slightly less than a 4m x 4m square, that's basically a small flat (larger than the smallest legal flat in a lot of developed countries, in fact)
While I’m just using data from the web and not actually measuring the spots in the parking lot atm, the basic size of a parking spot is 8x16 feet (128 sq feet). That works out to just over 11m^2 if Google can be trusted with the conversion.
Larger spots can go up to 10x20 feet which would work out to just over 18m^2.
And a 4mx4m room is not a small flat, that’s a small room in a small flat. But we may have cultural differences in what we determine small here.
I truly don’t think I can believe this. That’s exponentially tiny, smaller than my childhood bedroom and that room was 10x5ft (3 x 2) which was a single bedroom, not having to accommodate space for a toilet, shower, kitchen, counters, etc. I barely had a dresser and a twin bed in that room
The smallest living space you're allowed to put for sale/rent in France is 9m², and those absolutely are a thing. They're very common for example for student housing, but also former maid quarters, especially in older buildings.
The English Housing Act considers that one person living in a 70sqft room makes it legally "overcrowded", although there aren't any legal minimal sizes to sell or rent a flat.