HSN Summary in builder case

Yes — HSN/SAC summary is required even if in March the builder has only received advance and no sale invoice for flat is issued, provided the advance is for a taxable service supply such as an under-construction flat / construction service. GST Portal FAQs say tax is payable on advances received for supply of services and such advances must be reported in GSTR-1 in the month of receipt. The GST Portal user guide also describes Table 12 as the summary of all outward supplies HSN-wise/SAC-wise.