Please check whether you works fall under the definition of works contracts or not. If yes, rate of GST is 18% for works contracts. Please note that there is some relaxation in GST rate for works contracts done under governemnt project. so please check whether your project eligible for relaxation or not.
If it doesn't fall under the definition of works contracts, it will be treated as a mixed supply. if you do not raise the bill seperately, supply which attracts higher rate will be applied on complete transaction.