Ask an Property Solicitor. Get an Answer ASAP.
Thank you for your continued patience. I am on your enquiry now so you should get my guidance shortly.
Thank you for your patience. Although it is not illegal it would not be recommended for the tenant to deal with anything involving boundaries and access because in case of any damage the landlord will look to the tenant if they were not approached to give their own consent. Sometimes that temporary storage may have implications for the landlord's insurance in the event someone was to break in and steal whatever is stored on the property. So really best practice is to tell landlord.
Thank you for allowing us the opportunity to review your question. We are always glad to assist and to clarify anything for you. Do not hesitate to return and request @Solicitor4All to enable me to see your question without delay.