Quoting: EDGE
if its been there for longer than 4 years i think your good to go...
Nope, not as simple as that, either. I think It's 6 years, but I think you're still liable.
The guy would need to take out a (one off) insurance policy to cover the extension to cover it in the event of it needing to be demolished.
Even then, the new owner isn't covered for the loss / loss of value on their house, merely the costs.