The seller can sign the document as consenting witness since there was a sale agreement already entered by him with the builder and now this execution of sale deed would be based on a tripartite agreement.

Therefore the seller may have to be present to sign in the capacity of consenting witness while the builder is executing the sale deed before the registrar concerned. 

The building is under construction hence there is no chance for the builder to have transferred the property in favor of the seller by a registered sale deed, therefore the seller is not having title or interests in the property neither he is an owner, therefore the builder has to execute a registered sale deed in favor of the current purchaser.

Yes if any other fresh deed is registered then the presence is required otherwise not

What about transfer charges levied by builder on sale of flat 

seller has to bear the transfer charges 

seller also ought to have obtained no dues certificate from builder 

Seller is not legal owner of flat 

registered assignment deed is valid 

- An assignment deed is a conditional sale, and it is treated merely as licence , and the property can be revert back to the assignor as per contract , and the transfer of property only by execution of Sale deed gives full right to further sell the property . 

- Yes, the presence of seller/assignor is needed at the time of registration of flat done between the builder and the buyer.

An assignment deed can be viewed as a conditional sale arrangement, essentially functioning as a license rather than a definitive property transfer. Under this agreement, the property can revert back to the assignor in accordance with the terms specified in the contract. It's important to note that the complete and unrestricted right to sell the property can only be obtained through the execution of a sale deed. In practical terms, the presence of the seller or assignor is required during the registration process when a flat transaction occurs between the builder and the buyer. This ensures that all parties involved are legally bound by the terms and conditions outlined in the assignment deed, safeguarding the interests of both the buyer and the seller.