For my views, it is my understanding that said builder (though registered at Haryana) is having land in Delhi. He is constructing building on such land and then, selling under-constructed property (i.e. in the building under construction) to multiple buyers.
Without going into requirement of separate registration at Delhi or not, I want to explain why I hold a view that builder must take separate registration at Delhi. And my reasons are as follows:
A. Said builder must be availing various services such as 'architect, painting, plastering, civil work, works contract services and so on' from third party vendors for constructing the building on land located in Delhi. (This is general practice/s of builder-industry I noticed).
B. In terms of Section 12 (3) of the IGST Act, 2017, 'Place of Supply' of all such received will be 'Delhi' even if location of service-recipient is located at Haryana.
C. Input-Tax-Credit of gst charged by those vendors will not be available at builder's Haryana registration (i.e. at-least gst portal will not allow it). And for this view, I am not going into legal correctness or otherwise of the portal-restrictions in this regard.
D. To avoid such loss of ITC, it is in business interest of builder to take separate registration at Delhi.
E. Non-taking registration at Delhi will always be risky for builder as 'Delhi Tax Dept' got decent grounds to hold a view that that registration at Delhi is compulsory specially when land is owned by builder, when he is selling under-constructed premises on such land and considering generally known business-model of any such builder. And Delhi Tax Dept. indeed notice and take such stand, this can lead to multiple complications threatening continuity of ongoing operations carried at Delhi site as well as huge tax controversies / litigation on multiple grounds.
E1. Even taking favorable advance ruling from Haryana will not help considering wordings of section 103 (b) of the CGST Act, 2017. If any really wants, one should take such ruling from Delhi advance ruling authority. And experiences tell me that builder is unlikely to get favorable ruling on this issue from Delhi advance ruling authority.
(Kindly treat these as strictly personal views of mine and not as a profession opinion or advice).