General information on the best practice for the storage of fertilisers can be found on the Storage of fertilizers webpage and on the Safe use of fertilizers webpage.
Specific information on the legislative requirements when storing ammonium nitrate can be found on the NAMOS regulations webpage.
The AIC have recently published a booklet on The Storage, Handling and Transportation of Ammonium Nitrate-Based Fertilisers which can be downloaded from the AIC website.