The novel primarily takes place in the Verinders' house in Yorkshire and their house in London. But geographical setting isn't actually all that important in this novel. Sure, the characters are always flitting about by train, going from the Verinders' house in Yorkshire to London and back again. (Yorkshire is a region of England to the north and east of London, so it's not actually an easy trip.)
But the actual location of the house isn't as important as the house itself: the layout of the rooms, the floor plan, and where the different characters are in the house at different times is way more important. Gabriel Betteredge, the steward, is in charge of making sure that everything, and everyone, is always in the right place at the right time in the house. This is why he gets so "unutterabl[y] astonish[ed]" when he sees Rosanna in the library in the afternoon, since "after the library had been swept and cleaned in the morning, neither first nor second housemaid had any business in that room at any later period of the day" (188.8.131.52).
Still not convinced? Think about Miss Clack's eavesdropping scene. She is at Lady Verinder's house in London, and is asked to wait in the library before going to see Lady Verinder in her sitting room. Instead of staying there, though, she tiptoes through the house. She claims that she's sneaking around to leave religious books for Lady Verinder to find later, but we know enough about her hypocrisy to suspect that she's just being nosy. She ends up hiding behind a curtain in the drawing room and eavesdropping on Godfrey Ablewhite's conversation with Rachel. This isn't just a plot device to allow us to overhear, along with Miss Clack, what goes on between Godfrey and Rachel. It also shows us what kind of person Miss Clack is. She is the kind of person who isn't in the right place at the right time – she has no respect for boundaries, either literal or figurative.
Breaking boundaries within the home are part of what makes the theft of Rachel's diamond so terrible. It's not just about the value of the missing jewel; it's about the fact that someone entered her private boudoir (a sitting room adjoining her bedroom) in the dead of night, without her permission!
If you're interested in the layout of the Victorian home and the social importance of keeping things in their proper places within the home, you might check out the book, Inside the Victorian Home by Judith Flanders (see the "Links" section).