Yes, EXTENSIVE is a Scrabble word!

Letter values in Scrabble US: E 1, X 8, T 1, E 1, N 1, S 1, I 1, V 4, E 1.

Is it a valid word?

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

large in spatial extent or range or scope or quantity
broad in scope or content
of agriculture; increasing productivity by using large areas with minimal outlay and labor

Also: across-the-board, all-embracing, all-encompassing, all-inclusive.