Don't do the AC thing, it is a stupid trope under blogfluencers. There are no restrictions (besides positioning the outer unit in such a way that you cause your neighbors to lose sleep). As the summers get more extreme in Europe, more residents decide getting one is starting to pay off, so you see more AC's, but many people think they are doing fine without.
I can easily google restrictions and share them, and I have in other comments but let me throw it back at you.
Why do 90% of Americans have AC while only 20% of Europeans do?
Why does US have ~4 heat related deaths per million while Europe has ~235 per million?
Do you think it's just stupidity (Europeans don't know the relationship between heat and AC)? Or poverty? Any other explanation?
Yeah, never heard of such a thing. The restrictions are placing the units in common areas of the buildings -- in that case you need permission -- and external walls are usually common parts. Placing them in the façade may have additional restrictions.
But, if anything, energy efficiency standards for new construction are so strict that heat is becoming less of a problem.