Hi Mac74,
With regard to planning permission, if it is to the rear of the property, no planning permission but this is also dependent on the size of the extension can't remember exact square meterage. If the extension is visible from the road, yes you would need planning. Certificate of compliance comes into being if you ever need to see your house. If you go ahead and build now and don't get a certificate from Architect or engineer, it may prove difficult in years to come. My advice get the Certificate of Compliance now and avoid any difficulties later.