demesne vs countryseat

demesne and countryseat both are nouns.

Word	Noun	Adjective	Verb	Adverb
demesne	Yes	No	No	No
countryseat	Yes	No	No	No

As nouns, countryseat is a hyponym of demesne; that is, countryseat is a word with a more specific, narrower meaning than demesne:

demesne: extensive landed property (especially in the country) retained by the owner for his own use
countryseat: an estate in the country

demesne (noun)	countryseat (noun)
territory over which rule or control is exercised	an estate in the country
extensive landed property (especially in the country) retained by the owner for his own use