It shouldn’t be up to the purchaser but the developer to arrange some sort of landline service , it doesn’t necessarily have to be Openreach, but OR do have the largest network and work with developers of all sizes , provided the developer gets in touch early in the build, OR will build network into the property ready for occupation, but ( obviously ) OR can only respond if the developer gets in touch.
If your developer hasn’t made any arrangements with a network provider ( as you are posting on a BT forum , it would be Openreach as they are the only network provider that BT use ) then it’s very unlikely that you will have access to Openreach based services, FTTP or copper based FTTC/ADSL.
If the surrounding area is served by telegraph poles , it may be possible to reach the new properties if there is line of sight from the pole and it isn’t too far to span a wire, but that may not be possible, and it’s likely that the addresses, once they are registered with the appropriate authorities, won’t be matched against any usable network should you enquire with BT .
You should get the developer to make arrangements to get service provided , asking for ‘fibre’ may be confusing, ask what telephony arrangements are in place.
