I always thought it sorta funny that the Old House in HODS was rather larger than than Collinwood (if you go by the size of the locations used)...
On my first trip to Tarrytown in 1990, I drove around until I found Beechwood. It was then called Beechwood Estates, and the main house had already been divided up into (4?) units. They hadn't yet built the new buildings that are down at the end of the front lawn.
I know a woman here in Chicago whose old college roommate's parents have one of the units in Beechwood. She's stayed there on visits a few times. When I said I've always wanted to see inside the place (even though I believe the main entrance has been modified drastically), she told me she'd give me the phone number of her friend's parents. She said they're wonderful people and love having people visit them, but I just could not bring myself to impose on total strangers like that.
Anyway, when I was there in 1990, I spotted a caretaker and chatted with him a couple of minutes. He was unaware that the house had been used in a movie 20 years earlier. (Besides HODS, it was also used in an Ivory-Merchant movie called SAVAGES which I've yet to see.) I asked the man if it was okay to walk around and take pictures of the exterior and the grounds, and he said to go ahead, just as long as I didn't walk off with the place!