Here in English, we use the term House, in a somewhat formal and antiquated way, to describe a noble family. For example, the Queen of England from the House of Windsor. Or, in contemporary fiction, there is the whole war between House Stark and House Lannister.
In Spanish, does the word Casa play the same role? Or does casa simply refer to a domicile?
Only “de [House]” is not a surname in the usual sense, and conversely, a “de” surname doesn’t necessarily imply a noble house (either higher or lower nobility). Normally the “Casa” in the sense of the family is reserved for the higher nobility (cf the house of Alba, whose current matriarch’s lastnames are Fitz-James Stuart Silva; Silva is the second); for lower nobility same as for commoners, speaking of the casa de [lastname, not House] would usually refer to either the actual building or the household (that is, the servants), but then, for lower nobility the title would very often be linked to the building.
…and BTW neither is an “…y…” in the surname a clue as to noble lineage. Royalty uses it to indicate interdynastic marriages (Prince Felipe, “de Borbón y Grecia” because Queen Sofía is from the former Greek royalty) but lower nobility and commoners are all over the place on this as well.
The “y” is just a marker to indicate “the second lastname starts now”, it used to be commonplace but fell out of favor with the advent of record-keeping methods that made brevity desirable. There are a few cases where it’s still useful, such as when the first lastname is that “Fitz-James Stuart” which at first sight looks like two lastnames.
So what’s the etymology of such use? Why does the word for the building in which someone lives also refer to a royal ancestral grouping in so many languages?
That’s a good question. I’d be interested to know what the OED thinks. All I’ve been able to figure out is that such usage is at least a thousand years old. It’s obviously linked to the idea of “family,” but why this same metonymy in so many different languages?