I don't remember what the definition of a 'building' was in SL, but in ASL a building is well defined in the index & chapter B, and the rout rules are clear in that a broken unit does not have to claim the building it currently occupies as a rout destination. So, I would check the SL index and see what a 'building' is defined as, and cross check with the rout rules and see if there are any special rules, as above, in effect for broken units & routing related to buildings.
Sorry I couldn't help you more, but I stopped playing SL in '85ish when ASL came out & I simply don't remember the old rules. The problem with SL is that each expansion game COI, COD, & GI added rules & deleted supercerceded 'older' rules making it virtually impossible to play with one complete set of rules. I actually remember at one point, me and a friend were cutting up sections out of all 4 rules trying to eliminate the superceded rules & create just one rulebook .. it never worked. Best to switch to ASL, or use the ASL rules. Most (like 90%) of the old SL scenarios have been converted to ASL & republished, so you can easily play all those old scenarios (even using the old counters if thats all you have). You can find first edition ASL rules floating around on eBay quite cheaply.
