There is a high chance that you are not a self storage company's first customer and there were many people before you who used the unit. You have completely no idea what the guy before you stored in the unit and how is the hygiene standard. Not only that, is the self storage unit in an acceptable condition? Before you move into the storage space, make sure that you get all these nitty-gritty details sorted out.
Exterior of the storage unit
The exterior of the storage unit is very important. Are there any cracks or peeling paint on the walls surrounding the unit? Wall cracks if left alone will lead to water leaking into the unit over time. Unwanted pests like cockroaches and lizards can always sneak into the storage unit, leaving behind their faeces and possibly ruining your things. Also make sure that there are no rusting on the metal components outside the unit, such as the door hinges. The metal components may corrode and degrade the structural integrity of the hinges, causing more problems.
Interior of the storage unit
After making sure that the exterior is good, it's time to head inside. Make sure that the interior is in an acceptable condition as this is where you will keep all your things. Are the walls in good condition? Since the interior is protected from the natural elements such as the rain, it should be in an even better condition compared to the exterior. If it looks worse, something is really wrong with regards to the maintenance and you should talk to the owners. Are the metal rollers starting to corrode or are they kept in a pristine condition? Are the rollers able to slide up and down smoothly when you raise and lower them or are they constant jamming?
Depending on the type of storage unit, some standalone ones may have old and rusty roofs. Roofs are extremely important and poor ones will lead to water leak when there is rain. Old roofs are also vulnerable to break-ins, causing security problems. Try to seek access to the roof when you are moving into the storage unit and make sure that the owner clears out all the junk on it, including dead leaves and branches. Take the time to make sure that the roofs are not corroding and the sealants are still in good condition.
The above 3 points are the most important things you need to check before moving into a self storage unit. You do not want to be faced in a situation whereby you take over the storage space, only to realise that the condition is so bad and everything is falling apart.