Sez exemption to sub contractor

Our Company got a project in SEZ area for construction of building under sub-contract basis. SEZ client awarded this contract to contractor "A". Subsequently Contractor "A" awarded this contract to us. We are billing to Contractor "A". Whether we are eligible for GST exemption while billing to Contractor "A" or We can take exemption after making payment to Govt. What are the documents for claiming exemption? Exemption is available to Sub-Contractor who is taking contract from main Contractor? Please Clarify