(Yes, I know. The settings isn't persistent. Will see if I can get around for a quick update for that later. Until then - modify line 120 yourself.)
[attachment=2:29zligje]<!-- ia2 -->Flatten-Extrude.png<!-- ia2 -->[/attachment:29zligje]

I think I'm missing an important step... neither flatten nor flatten + extrude works for me. I've tried it on a 37 and 97 meg file, follwing your instructions exactly. I'm not sure what I'm supposed to enter into the layer filter though. The layers I need to include are called "S-BLDG and S-TRAILER", they are all by default under Layer0... I tried separating them with the pipe/vertical line and everything with no avail. And where is line 120? Please help... this would save me HOURS!

With that said, I think you are doing a great thing with this tool! I'm an architecture student and this cuts some serious time and I can't wait for your roof app... although I have to say that most topo-scans are much "dirtier" than what you have. Mine don't even have any indication of where gables would/could be...

Props and Thanks!
(Sketchup 6.0515)