The GST rate and value of supply depend on the manner you invoice.

In fact, there are two ways to invoice.
1. Invoice like a pure agent:
- You should explicitly mention the hotel room rent, conference hall rent as separate line items on your invoice. And write the actual charges that you paid to the hotel in the value column.
- You should explicitly mention your fees/commission in a separate row of invoice.
- In this case, you should charge GST @ 18% on your fees only.
- Depending on your location and place of supply IGST or CGST+SGST would be charged.


2. Issue a bundled invoice.
- You can mention all the services (like hotel room rent etc.) and not disclose their cost in the invoice.
- You can specify a single value viz your total charges + hotel rent etc.
- You should charge GST on your total fees (i.e., the total bill amount).
- In this case,
- you are offering a composite supply, so GST rate that hotel room charges would be your GST rate.
- You can claim ITC too.

All that said please note that:
- You could follow different invoicing ways for various clients.
- Classification as regular supply or mixed supply or composite supply will depend upon the services that you offer.
- There could have been a case that you don't have to charge GST even. It is possible if your aggregate turnover is less than Rs20Lac(or Rs10Lac) and if you are not a registered taxpayer.