Following up on Estate Ledge; go up the stairs to the railing, jump on the railing, turn left and jump onto the crossbeam as was stated in that post.

From the left crossbeam, jump onto the tiny (game inches wide) ledge that runs down the length of the room (it isn't obvious that it is there) and hang out in the corner. You can shoot down on anyone who comes into that room and you can watch the back entrance ramp to the upstairs quite well.

Retaining wall between the house and the waterfall; If you come up from the garage with the hole in the roof, past the waterfall area, go up the reddish path to the side of the retaining wall. At either end is a bit of masonry at the bottom edge of the wall. If you stay very close to the wall, you can get around the wall on either end.

Also, from the side door of the greenhouse (at end of retaining wall) you can jump up the space between the retaining wall and the green house. You will need to either have a submachine gun/sniper rifle (for high mobility) or have a Marathon/Lightweight class.