On 6 April 2009 new Planning Laws came into force.
Before the new Law, the Environment Department (formerly the IDC ["the Department"]) was not able to prosecute a successor in title (the new owner) for any work that a previous owner had carried out without planning permission. The Department could only prosecute the person who actually did the work.
Under the new Law, the Department can take enforcement action against the present owner for unauthorised works that were carried out after 6th April 2009, whether they did them or not.
To protect you from being prosecuted by the Department, when you are purchasing a property you can apply to the Department for an Immunity Certificate ("the Certificate"). Provided that the Department are not aware of any breaches of planning consent at the time of the application, they will issue the Certificate in your name, which will act as a statutory defence to any proceedings they may bring in the future for unauthorised work by a previous owner. It will obviously not act as protection for any unauthorised work that you carry out.
It is not a legal requirement for you to apply for the Certificate, however it is probable that if you are borrowing your lending Bank will insist on it anyway. The cost of obtaining the Certificate is £50.00 and for the protection that it gives you, it is worth the money. The Department will process the application for you within in 7 days, so it should not slow down the process of buying (providing the application is successful), but the sooner the application form goes in the better, to avoid any hold-ups.
Your Advocate/Clerk will advise you how to apply for the Certificate when they contact you.