Service tax on electricity and water charges in works contra

My client is a civil contractor who under takes construction/repairs/interiosr of commercial/factory buildings

in some cases the electricityrand water is supplied by the client 

1. In this juncture can service tax authorities increase the contract vaule beyond our agreed terms and charges service tax in the difference amount ?

2. If it can be doen what is the percentage of vaule to be increased ?  (i,e, how to quantify the amount involved)

3, And  what will be rate

4. is there any judicial orders on this

thanks in advance