How do I convince my husband to move out of our shared closet, and into a standalone closet?

We live in an old home with few closets, what closets we do have are extremely small. In our master bedroom is one small closet we share and it's not the size that's meant to be shared. His side is always a mess and I can't ever get into my closet. It is making me crazy! I don't want to continue living like this and I don't want to nag either. I will buy him a standalone closet system to put in our finished basement if he will move his clothes there. How could I convince him this is something I need and he will ultimate enjoy?