GST on works contact

Note that GST is payable on advances. So, Interest is applicable from 2017 even work is started or not yet started.

If you say it is a Security Deposit, then GST not payable on the same.

In case of Services, GST is applicable on advances and Provision of Section 13 shall be applicable. Accordingly, Liability to pay GST Start from very first day of Receipt of advance. Hence Interest shall be applicable after the due date of filling of GSTR3B Return of respective month