Race: Unaligned/None
Cost: You decide

It's a creature that enchants a building.
The building is still a building but also has a living counterpart.

The building gets X points to distribute to any of its attributes, adding no more than 2 or 3 (pick a number) in any. (e.g. if X=5 then +2 max hp and +3 vision; or 2 attack, +1 vision, and +2 range; or +2 speed, +3 range)

Immune to Charm and Possession.

Soul Plague and Lysis and other cards that only destroys units will destroy the symbiont (and the bonuses) but leave the building intact.

It sounds like it would be a Sylvan card being a living thing, but it is a symbiont and can't be picky about who its host is (you can cast it on any building)

Symbioses sounds very interesting but your description makes buildings too strong. Even if I'll add only three points to Earthworks (+1attack, +1range, +1sight) it will be Very strong with all MF stuff like repairing. Actually there's "Leg's of baba Yaga" card that give partially same bonuses.
It's very good idea in general, but not your's variant.

So is the idea that the symbiont can move around independently of the building?