The below screen captures show us the house grounds, including original gated entrance off Willow Glen Rd., the familiar walkways (still there today), and the exterior of the original house.
The follow screen captures show the inside of the original mansion, and are the only images I've ever seen of house interior. I doubt any set dressing was required. The house had been condemned for ten years and probably looked exactly like the abandoned ruin it was portraying.
You can buy Mannix Season Two, which includes "Fear I To Fall," at Amazon.
Thanks to DM for the tip and the screen caps.