Gst service bill to sez

Please  refer  section  16 of  the  IGST  act  2017  and you have to act  accordingly . 

IGST will applied in all cases if supplies is made to sez.... supplies to sez is deemed to be export

The transaction falls under IGST

Dear Narinder :
As per Section 16 of IGST Act supply to Sez is treated as Zero rated sale (Export sale )

So need to charge IGST on bill to Sez.
Else file Lut as per Rule 96A online on portal form RFD -11 & no need charge IGST on export invoices.

Now term of Notification 15/2017 igst (r) , give exemption to Sez unit for any goods & services imported for authorised Operations.
So even though you charge IGST on IGST on bill (without lut) to Sez unit , the Sez unit will never pay you the IGST amount. you have to claim refund for the same as per prescribed in Section 54 of CGST Act .

So , You can charge IGST but sez will not pay you iGST Amount.