1) The GST tax input and New tax rates has to choice of builder and what he has to choose and get implemented accordingly. 

2) Actually from 01st April 2019 it should reduced to 5% for under constructing project or if builder wants to keep as old GST rates than the accompanying input tax credit (ITC) benefits.

3) “The new GST-related announcement has given real estate developers the choice to either opt for the old rates and the accompanying input tax credit (ITC) benefits, or else to adhere to the new reduced GST rate of 5% without ITC. "

Is applicable to whole cost of flat, As within next month on under construction GST will be cleared by GST committee more, for time being it has been kept in Builders hand which option he has to choose.

Don't pay 12% on entire amount if builders asks for it then send legal notice and file consumer complaint against him. 

pay the amount under protest 

take possession of flat and sue the builder to recover excess amount paid by you 

contact a local lawyer for filing case beforeconsumer forum 

Builder can  deliver possession only receipt of occupation certificate 

merely on basis of architect certifcate possession cannot be delivered by builder 

Builder's demand is illegal

Prior to 1.7.17 builder could charge service tax at 4.5% on all payments

On and from 1.7.17 builder has to charge 12% gst on the payments made after 1.7.17, however the quantum of the instalments was required to be reduced by builder on passing on benefit of ITC credit availed by him

So there ought to be a pro rata reduction in your instalments 

So it is illegal for builder to demand 12% on whole cost

Dont be intimidated by his threats of levying any penalty on delayed payment 

Make all your payments on a without prejudice basis only

Also architect certificate is not enough to give possession 

Builder has to obtain occupation certificate from municipal corporation and then offer possession

Also if it is case of builder that the architect certificate is as good as occupation certificate then tell him that if any payment is required to be made by you on taking possession then that will not attract any GST

So if he says that building is complete and OC is received then he has no business to charge GST and that too at 12%!

The builder has indulged in unfair trade practices against which you can complaint to Rera

For not passing on the ITC benefit you can approach the national anti profiteering authority constituted by government and lodge a complaint against builder with that authority 

For now issue him a very strict legal notice with copy marked to police station for indulging in fraudulent practices 

You can file a complaint in rera and seek compensation 

Dear Client,

Demand is illegal, complain to RERA and if any amount payable now than on revised GAT rates which is further lowered.

you first write to them demanding all the details about the taxes they have charged you and the taxes that they are demanding now and can state that you require these details urgently for filing income tax returns.

If they refuse to furnish the proper details , you may issue a legal notice demanding the details after which you may prefer to  raise a complaint with RERA about their  exorbitant or fraudulent claim in the name of taxes or can drag them to consumer forum on the same grounds and seek compensation also for their mental tortures.

The builder may play many gimmicks like obtaining a certificate from an architect etc., since you are the buyer you may insist on OC or CC before taking possession or getting the property registered on your name and also can keep a hold on the payment of the balance amount. But you should have a written correspondence that why you are holding this amount so that you can legally fight it back when it comes to the legal forum as a dispute.

Dear Sir,

- Do complaint to RERA Tribunal against Builder for delay and illegal charges.

- MRERA shall take the cognizance against Builder illegal activities

Regards

Vivek Arya