Gst impact on price of apartment

Dear Experts,

I need help on understanding GST impact on price of apartments. My wife and myself are jointly buying an apartment. Builder have bene sending us demand letter as per construciton schedule, and we have bene paying it. 

Demands till June had about approx. 5.8 Lacs (10% of total cost), and builder asked us each time to deduct a TDS of 5800. We had been paying TDS Rs5800 and a Service Tax of approx R 26000 directly through 26QB for installment.     

Now after GST, the last demand letter in July for next payment has asked for next installement amount of 5.8 Lacs, but is asking us to pay a service tax of 69,844 (GST @ 12% of 582036). 

My question is - why is such a huge leap of service tax from about Rs 26000 to Rs 69844? I understood form some internet articles prices should actually be reduced   (https://timesofindia.indiatimes.com/business/india-business/gst-gains-prices-of-flats-may-drop-by-up-to-5/articleshow/58798164.cms)  

Am I missing something? Is the builder demand correct? or are they not passing on the GSt benefit to us i.e. buyers?

Thank you very uch for your helps.

Regards,

S. Das