I guess you mean you bought the house with the extension already there and now you are selling.
There are two parts to this - Planning Permission and Building Regs approval.
Planning Permission may have been required if the extension is large, but often one can use what are called Permitted Development Rights to extend. And anyway if the extension has been there more than 4 years (and it needed Planning Permission in the first place), no-one can now force its removal. In the worst scenario you may be asked to acquire a Certificate of Lawful Use.
Lack of Building Regs approval is likely to be more of a problem because there's nothing to show the extension was constructed with adequate strength in foundations, with adequate insulation, with ventilation - that sort of thing. And you can't get retrospective BR approval. However the good thing is that it is still standing - so there isn't probably a problem. However that doesn't stop sharp-eyed solicitors demanding an indemnity insurance policy - just in case a problem arises. Such policies cost a �100 or so.