Yes, DEMESNES is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, M 3, E 1, S 1, N 1, E 1, S 1.

Is it a valid word?

DEMESNES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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

Also: landed estate.