Can earthing of a building degrade over time
Earthing can be damaged.

Whole earthing installation needs to be continuosly led to the ground, if something (a tree branch) or someone (a worker fixing house's external surface) brake the line, it needs to be checked and fixed.
My parents' house have had a similiar problem some years ago. They were making additional thermal insulation on external surfaces and, after deteatching wire leading to ground rod. Fixing the continuity fixed the problem. 

My first guess here is to check the connection to ground rods. I don't think that it can be 'degraded', 12 years are not that much, I guess.