Any fics where spike has a huge house/mansion or aristocratic/old money estate that Buffy ends up in somehow. Employed, living, kidnapped, party, visit, whatever. Any period.

How about the house we see William living in with his mother in Fool For Love and then again in Lies My Parents Told Me. Would that count as fancy? Most time travel fic that have Buffy going back to 1880 and live with William would meet your request in that case. Off the top of my head, there are

-Forward To Time Past by Unbridled Brunette

-Only Time Will Tell by Dampersandspoons

-Yours William by puddinhead

«Touched by heaven» by Loup Noir on EF, I quote: "Why do you live in a crypt when you've got a mansion like this?"