I'm trying to buy a Grade II listed Georgian townhouse with a rear extension consisting of a ground-floor utility room with first-floor bathroom above. The vendor has no knowledge/proof of when this extension was built. Our surveyor says the bathroom (first floor) was probably built pre 1950 (when the property was listed 
) and the ground-floor utility room added later 
.
My question is: how do I ascertain that this extension is legal? Due to the fact that there is no time bar on enforcement action, I am concerned that if I need to repair the extension (likely) and apply for LBC to do so, the conservation officer may then say that it contravenes the listing and could insist on it being demolished.
The vendor does not seem keen to approach the council to obtain retrospective LBC for the extension. Our solicitor has suggested indemnity insurance but says that if at any point in the future I have to apply for LBC in order to carry out repairs, this would void the policy.
Has anyone been in a similar situation and did you find a solution? I LOVE the house but am wondering whether I should walk away . Without the extension it would be too small for my family.
TIA for any thoughts 
PS Going out for a while but will be back later.