Tenant wants to make basement livable space...

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I recently purchased a new duplex with an existing tenant (on last month of lease). I am in the process of writing a lease for my use. The current tenant has mentioned that he would like to make the basement (which is exposed in back with just a steel door - no windows) livable space for himself. I have a clause in the lease about no changes without written consent. To me, it's a basement - use it for storage, laundry, etc.. but it's still a dirt floor and cinderblock walls. Does anyone have pros - cons, thoughts on this issue? Is there something I'm missing... thanks.