12 x 20 represents the outside dimensions of the pool walls. The braces and top ledge will extend beyond this. You'll need at least 18" beyond the pool walls all the way around. This would make the amount of space needed closer to 15 x 23. These and other additional measurements can be found starting on the 3rd page of the manual which leads nicely into the second part of your question. Yes, all of our pools come with an installation manual. If you'd like to view this manual now, navigate from this Questions & Answers tab, to the next tab over which should be Resources. You can download the manual from there.