Sale of agreement for apartment and delay in construction

Ordinarily in cases such as yours, 3 parties are involved. The land owner, the builder and the buyer. Since the builder concieves the project, the land owner ordinarily gives power ot attorney to the builder to register UDS in favour of the buyer on his/her behalf. The builder then enters into a construction agreement with the buyer. Legally the builder's role is that of a construction contractor. He is to construct as per the specifications and terms mentioned in the building/construction agreement with the buyer. If the land owner/Builder (as his POA) has already sold all the land (in the form of UDS), there is no further space to offer to additional buyers. In simple terms, all those buyers who have bought UDS as per the approved plan hold collective ownership over the land where the apartment block is being put up. 

Read carefully the terms of the builder's agreement that you entered with the builder

The buyer may sue the builder for specific performance of his obligations if the terms of the agreement so provide.

If there is any penalty clause for delay in the construction schedule, you may invoke that provision against the builder to seek monetary compensation for delay.

In extreme cases, after carefully considering the economic implications, the buyers may collectively rescind the construction contract.