Forget the topic of double parkers. I have lived in an apartment block with such for over 10 years. Outside on the street you rarely find a parking space but the underground garage is rarely more than two-thirds full. Such a thing gets big. Length and width are the smaller problem, the real issue is the height. To properly lower the car underneath you have to dig deep. If you don’t do that, you get crap like with us, with 1.50m for the lower car (I bought the Octavia with 1.48m including roof rails exactly into that) and a long car won’t fit above. If you do it right, you easily dig 3 meters deep and still have a 4m high garage above. There is the option to save some height by tilting, but then you have the trouble with the slope angle. At our place some ramps have been riveted to the parkers so the car doesn’t bottom out. And I only managed to get my trailer on with a ramp. These things are also quite maintenance-intensive and in winter salty water always drips on the metal sheets which after about 30 years all rust through. Unnecessarily, I would never put such a thing in my place.
Get an appointment at the building authority. 3 spots per residential unit are a bit excessive, maybe they can be negotiated down to 2. But the problem is that you only get those as trapped parking spaces and that’s totally impractical. That’s why it is often not accepted.