Yes, MAISONETTE is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, I 1, S 1, O 1, N 1, E 1, T 1, T 1, E 1.

Is it a valid word?

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

a self-contained apartment (usually on two floors) in a larger house and with its own entrance from the outside
a small house

Not playable in Scrabble US: MAISONNETTE.