I've been doing a fair bit of research on offsets lately in order to dial in the look that I'd like to go for with my wheel purchase for next year. This is a good read.
http://www.turnermotorsport.com/html...clopedia.shtml
"Most BMWs will take a 15-20mm spacer when used with the stock wheel/tire setup. This is because the German TUV (similar to our own DOT) still requires adequate clearance for snow chains. These are still widely used in Europe, especially in The Netherlands, Norway, and Sweden even though they are obsolete or irrelevant in other markets. The TUV also requires that every car in Germany be equipped with either dedicated snow tires or snow chains for winter driving."