The house was built as a standing set in Concord, Massachusetts. It was torn down soon after filming ended.
When Newton first enters his house and finds Gwen there, she is singing "The Name Game". Goldie Hawn uses the names Ollie and Kate, both names of her children in real life.
Meg Ryan was originally set to star as Gwen. When Ryan declined, the producers opted for Kim Basinger. But Basinger also declined, so Goldie Hawn took the part.
At the 1:40:30 mark, Newton is confronting Gwen about how she came to be living in his house. Goldie Hawn walks from the kitchen into the living room and Steve Martin follows, tripping on something, somersaulting over the couch, and landing half on the floor and smacking the coffee table. Right before he pops up, you can see Goldie Hawn look off-camera and smile - apparently his stunt was more than she expected.
In Taiwan it was known as Half a Wife in My Soup.