Tenant asking to do a walk in closet for a room

[Sandy Gabin]

Hi BP family! So one of my tenants asked me if I can let her build a walk in closet. I told her that I would think about it. I feel like there could be a red flag.  She is a very good tenant but I didn't want to say no right on the spot.  Any advice? If she is asking is because she is going to pay for it.  I am open to any suggestions or advice or does anyone have any experiences?