consent letter for new GST registration

A partnership firm ABC having its registered place of business at the business premises of one of it’s partners say partner A( the partner A having rented premises for his proprietorship business). The partner A gives consent to ABC firm to use a part of his proprietorship office space without any consideration in return. In the state of KARNATAKA.
I would like to know for the purpose of GST registration of ABC
1.Can the proof of place of business be a consent letter from Partner A to ABC Firm along with electricity bill in the name of partner A?
2.If yes, does the consent letter have to be on a stamp paper ? of what value?
3.While taking the stamp paper what should be the descripttion/purpose of the stamp paper?
Would appreciate help from experts.Thank you in advance.