Any building or structure that will be used for housing or storing heavy items of equipment or machinery such as a riding lawn mower, tractor mower or zero turn mower should be built on a solid and level foundation.
When you have solid, level foundations laid down, then when you are assembling the shed it makes for a whole lot easier job. This is because the walls will line up with each other, the bolt holes will line up and when it’s all slotted together the door should fit snugly and the roof alignment should be perfectly level.
And providing that your shed has a strong suitable heavy duty floor installed then there should be no problem storing heavy equipment in there.