Subletting

Tenancy Types and Management

Hello
I have a decent tenant who is on a 6 month AST which comes to an end in a month or so. I am happy to give him a new lease and he would like to stay. He has asked me to consider allowing him to sublet the second bedroom as he would like some company and would prefer to stay rather than moving into a house -share. My question is : what are the implications of agreeing to this and if so, how should I go about it to ensure I dont fall in the cart somewhere down the line either with the tenant or the prospective other. Many thanks.