Thanks for your enquiry.
I am afraid that with any new build purchase (or any purchase for that matter), the Seller/Developer prepares a Plan showing the extent of the property/land they are selling. It is good practice for a Buyers Solicitor to send their client a copy of this Plan. As yours was a new build, you should therefore have been sent a Plan showing the location of the property itself and any garden and other land which is included and which is being sold to you. This Plan is also attached to the Contract and the Developer is legally bound to sell you the land as per the Plan and nothing more and nothing less.
I am sorry this may not be the answer you were looking for, but it sets out the legal position.