Why is it on Universal's lot? Isn't Psycho a Paramount film.
Psycho was the last film Alfred Hitchcock made for Paramount. By the time principal photography started, Hitchcock had moved his offices to the Universal lot and that was where the film was shot.

Design: The Psycho house was built from a number of Universal's stock units to save money. The front of the house and the distinctive tower was the same as that used in the "Harvey" house on Colonial Street. As the Jaws photo below shows, the rear of the house used to be completely hollow - it was just a shell. The house was enclosed when it was moved to location #2 in 1983.

The house set has been moved twice since it was originally built.

Location #1: 1959-1982 (approximate dates) - The house appeared as on the right, on the hill above Singapore Lake (now the home of the Jaws attraction). In 1964 the side of the house visible in the photo below was added - originally the house was a shell with just two sides (front and left side) as that was all that was seen in the movie. The Psycho house was one of the main attractions of the early days of the tour.

Location #2: 1982 - ?1986: The house was moved to a more remote location in the pre-production period for Psycho II (1983). The area by Singapore Lake / Jaws Lake then became the site for The Chicken Ranch when it was moved onto the backlot following production of The Best Little Whorehouse in Texas (1982). The new Psycho House location was on a plateau at the south end of Falls Lake (before the log cabin was built). The fourth photo on the right is from the Halloween Knight episode of Knight Rider (1984) and shows a hazy Psycho House at the end of Falls Lake.

Location #3: ?1986 - present:
Adjacent to old Falls Lake (where the swamp scenes were filmed in the original movie) the house and motel have been recently restored. The house was in this position in 1986 when the Falls Lake backdrop was being built, and it's likely it was moved when this construction work started.
See current Psycho House photo gallery and Bates Motel photo gallery.

There's lots more amazing detail about the Psycho house at the
Psycho House & Bates Motel Timeline