The deplorable state it was in last year was due to a fire.
It wasn't owned by the Hesse family any more, at least not as far as I know: although Ernst Ludwig was allowed to keep certain properties after the revolution of 1918, Schloss Seeheim was not one of them.
In his memoirs, Ernst Ludwig mentioned some of the plans he had had for his country that were destroyed by the revolution. He had wanted to build a new mansion on top of the hill, according to his own taste, as a personal statement for later generations, as he had always preserved and embellished older, existing buildings built by others, but had never built one himself.