Your client may have a landlord's policy covering both buildings and contents and store some of their own contents in part of the property (e.g. the garage) that they let to tenant's.

Provided that the items fall under the policy definition of 'contents', the items would be covered by the policy.

If the garage doesn't form part of the tenancy agreement, i.e. the tenant doesn't have access to it, this doesn’t affect the building insurance which still covers the garage as it belongs to your client and forms part of the risk address.