Composition scheme service provider

Yes GST is leivable on advance received.

You have to calculate tax on advance and pay tax while filing the return for the month. The advance received should be grossed up. This means that advance received is considered inclusive of GST. When the rate of tax cannot be determined during receipt of advance GST @ 18% has to be charged. Also if the point of sale cannot be ascertained the advance is considered as interstate supply and IGST has to be paid.

Yes, GST is leviable on advance in case of supply of service. 

Adjust the advance received with the outward supply in future when you raise the tax invoice and makes tax payment against that invoice. 

Thank u sir

Your welcome