In early 1970's Constance moves to LA to pursue an acting career. It is unclear if she actually moved into the murder house at this time, but in 1984 Constance and her children move out of the Murder house and move next door. So at some point before this her and her husband move into the Murder house.
There is speculation as to why she went next door. Rumors suggest the house doesn't let people leave and only let her leave so she could "work for the house".
In 1993, Larry and his family move into the murder house and after Larry's wife discovers Larry and Constance are having an affair, she kills herself and her children. That's when the Langdons move back into the Murder house with Larry.
If you think about when Tate was shot by the police in 1994, he was in his room in the Murder house (which became Violet's room IIRC), NOT next door in Constance's current house. Why would he have a room if he wasn't living there.
So, Beau was chained in the attic because they in fact were living in the Murder house.
This was a great question because the timelines and events are scattered, vague and hard to piece together. I used a few different resources to come up with this answer. Here are the two significant ones