I am considering buying a property with a small extension that the seller says was built in the 1960s but there is no planning permission. I'm looking into this and not sure whether to request the seller obtains a certificate of lawful development. It would appear to me to fall under permitted development anyway as its single story the property is detached and it falls under 4m in projection and height. They have offered an idemnity policy but its not clear if this would cover the cost of taking it down, then re-building with planning permission. Is it unreasonable to ask they apply for a certificate of lawful development? The solicitor isn't very helpful they just want to push through the transaction.