follow your jurisdiction. I'd say just do a change in terms and conditions on the basic lease agreement already in place increasing the rent, and also then detailing what and where and dimensions. If tenant wants a what if I remove the stuff,....add reduction provisions. cover his insurance coverage in your change in terms and conditions. Make sure you're not going to be on the hook for thieft or vandalism of his tools and equipment,.....as that stuff is prone to disappearance !