I have mine outside while I continue to work on the garage, and this week I had occasion to use it. The driveway isn't level by any stretch of the imagination, and the lift works fine. While the manufacturer doesn't recommend using it on an unleveled surface, I haven't found any problems doing so. It might be more of a liability issue than anything else. In my estimation, a variance of about 3/4" + / - shouldn't be a problem under most circumstances. I am not endorsing doing anything that a manufacturer recommends, but I am merely stating what I consider safe for myself, and not anyone else. I am responsible for my own errors in judgment, and you also have to do the same for yourself.