Complex sentence: Cobham Hall, also in the highest architectural category, Cobham Hall is a stately home which was formerly the seat of the Earls of Darnley: since 1965 it has been an independent girls' school.
Split into simpler sentences: Cobham Hall, also in the highest architectural category, Cobham Hall is a stately home which was formerly the seat of the Earls of Darnley. Since 1965 it has been an independent girls' school.