First of all, you have to get on top of the perimeter walls and get on top of the swimming pool roof. Then (having your back to the perimeter wall) walk over to the very left corner of the roof; turn around so you are facing the perimeter walls. Then walk forward until you can walk no further, side step right (towards the mansion) and turn Lara so she’s facing the Hedge Maze, like she is going attempt a jump to it.

Then, just jump up and down (press only jump) a few times, well maybe more than a few times, but hold on jump and before you know it, you'll be on top of her roof, free to explore, It does take some time to get it to work, so be patient because it does get frustrating.