Lawful Development Certificates
There's some more words on them here that might help. If your buyer is going to pull the whole thing doewn anyway, there's no issue - he'd have to re-apply for PP for whatever he wanted to do. I think the timescale depends on how busy the Planning Dept are - PP decisions are supposed to be made in 8? weeks but many Authorities are failing to meet the timescales demanded by Government - make an enquiry at Development Control by phone?
http://www.planning-applications.co.uk/enforce ment4a.htm
Building Regs
You need to be careful if you are thinking about an indemnity policy because you certainly can't get one if you (or the buyer) have notified BC of the transgression. The reason is obvious - the policy is designed to indemnify in the event that BC decided to take retrospective action. And what they don't know about ...........
A second way in theory is to apply for retrospective consent. I don't see how you can apply for retrospective BC consent because with the original doors removed the "non-habitable" status of the room has been compromised and it is impossible to meet Building Regs stds on insulation because of the translucent roof - even though the main issue that people are generally concerned about regarding lack of BR approval is the structural one (foundations depth, lintels etc.)
But, again, if your buyer is proposing to demolish/re-extend, surely he won't have an issue with this?