Even when the 7 years enforcement window (12 years where planning has been granted) has elapsed, non-compliant developments can still be problematic. Any future developments might be blocked until such time as the non-compliant development is removed.
That said, this could well be an exempted development. A local engineer would be able to guide on that and provide a certificate of compliance. Most decent solicitors will look for certification that what's on the ground is planning compliant, so you might be best looking to arrange that in advance of putting it up for sale.