CommanderROR, I'm not sure what kind of wall construction they use in Germany, or anywhere else other than Canada, but here they use what is called
drywall. Large sheets of paper covered gypsum board that covers the inside of the house over the framing. It is then taped to cover any seems and then a thin coat of plaster over top to make it all smooth before priming and painting.
If drywall, or whatever you call it, isn't used, just ignore this post.
The old days of lathe and plaster are rarely seen here, becoming a lost art.
The reason I mention this is because sound-proofing between the adjoining walls of duplexes is often not adequate and can cause problems with your neighbors. Of course there is the usual soundproofing insulation that has been around for years (looking somewhat similar to external fiberglass/paper insulation). I have heard of friends buying one half of a duplex and having to deal with aggravating noise problems. If your neighbors are your friends, it would be better to deal with a soundproofing solution before construction starts. (
and you probably have and, if so, just ignore this post again).
There is a relatively new brand of drywall on the market that has a thin layer, or layers, of metal sandwiched in the gypsum board and can give you up to 10 times the soundproofing of regular drywall and way more fire protection. This, of course, is more expensive but you only have to use it for any adjoining walls. Builders are also using it here for houses and condos built near a lot of noise, like busy traffic, trains and transit systems.
This link might explain it better than I can. There is also a sound tester for each type of drywall and what sound from a stereo you can expect to travel through from your neighbors. (turn your speakers down if you try this). To learn more or listen to the difference this product can make on your walls try the following link:
http://www.quietsolution.com/html/quietrock.html
This whole suggestion may be moot if you and your friends are nice quiet people and this is totally unnecessary, or you have construction methods to deal with this sound-proofing in as good or better a way.
Just a suggestion. WDE.