I do believe the answer will lay, in what was happening with the vacant half, and what you propose to do with it. If it was available for lease, and this can be confirmed, ie advertising, sign, internet etc, and you are buying as a Landlord, and will be continuing to look for a tenant, then I understand that you purchased the whole as a going concern, and no GST should be payable. However if you intend to occupy, then there could be an arguement for GST to be apportioned. As I have not had this experience with a single title sale, I am not certain. If each portion has its own title, that would be clearer. I hope that helps, however get confirmation from a GST specialist accountant prior to signing the Contract.