It's not exactly a measure, but I recall Ken Thompson saying that good code makes it easy to change or extend a functionality. (I think it was in Coders at work

It's not exactly a measure, but I recall Ken Thompson saying that good code makes it easy to change or extend a functionality. (I think it was in Coders at work from Peter Seibel). If you want to derive a quantitative metric from that, I guess you could take the average number of lines/time required to change or extend something.