Can I add a shelf unit, will the floor hold?

Yes, you can install shelf units in a plastic shed. We do provide shelf accessory options for some of our Palram Canopia Sheds (please see the "Accessories" option underneath the product description). These are 3x1 shelves which can be attached to the inside of the unit (instructions supplied). We do stock free-standing shelves for sheds and garages; take a look at our shelving category.