Is GST applicable on services rendered to overseas clients?

1) Export of Goods or services are Zero rated supply as per section 16(1) of the IGST Act ( please refer) , however you have to pay IGST and Claim refund or submit LUT ( Letter of Undertaking) . 

2) Export of service should as per definition given in the section 2(6) of the IGST Act ( please refer) meaning thereby, you have to fulfill all the conditions in section 2(6) of the IGST Act , to consider export of service , and to take a benifit. 

Take more opinion on this. 

1. SAC code of a service remains same irrespective of whether it's domestic or export sale

2. GST number needs to taken if the threshold limit of 20 lakhs is crossed else not required. If you take a GST number, then LUT shall be preferable.