Blocked vents

Fixable, and not a reason to walk away on its own. Under Approved Document C the void beneath a suspended concrete floor needs cross-ventilation from opposite sides to clear moisture and ground gases. With only one rear vent left, the void will be poorly ventilated. Beam and block won't rot, but you can get damp air, condensation and, in radon-affected areas, gas build-up. The extension should have been built with ducted or telescopic vents carried through to its outside wall. This can usually be retrofitted by core-drilling and ducting through the extension floor. Before exchange, ask for the Building Control completion certificate for the extension, as an inspector should have flagged this. Have your surveyor check the void through the remaining vent for damp, and check the postcode on UKradon. Then ask the seller to reinstate the ventilation before completion or adjust the price.