Hi Da Vinci, speaking as a registered Architect, it sounds as if it is exempt but building on a boundary line can complicate matters and without seeing it, I could not be sure.
However, the onus is on the vendor to provide documentation for any extension or development of the house.
If it required Planning Permission, a copy of the grant of Planning Permission should be supplied together with an Architect's Opinion of Compliance with the Planning Permission and Building Regulations.
If it did not require Planning Permission, an Architect's Opinion regarding Exemption from Planning Permission and Compliance with Building Regulations should be supplied.
It is in your interest to ask for this as you will have no comeback after the purchase is complete without them. And if the vendor will not supply, you would have to ask what they have to hide...