Most of the time you can't because the city/neighborhood/building council wouldn't approve it. Even if you put it on your balcony which you would've thought is your property.
One of these things. Its just that they're so uncommon in Europe you've probably never even seen one.
At least in the UK (one of the countries with the least AC) you don't need planning permission for a home unit. I use a portable one, since I'm still renting, but permanent units are slowly getting more popular.