It's been fairly common knowledge that the only guaranteed safe storage containers are those which are inside houses that you own. However, Hearthfire adds storage containers that are both technically outside and also still your own property.
Are these containers safe to use? Does it depend on whether or not you've actually gotten to build a house on the property yet?
Best Answer
Any container INSIDE your house is guaranteed safe. On the other hand, containers OUTSIDE the house (i.e. barrels, food bags etc) is NOT guaranteed safe. This also applied to any house that isn't yours, although you can sleep in it (i.e. Alchemist's Shack). Exception for Hearthfire, those containers outside your house are safe storages so don't worry about losing anything.
My rule of thumb for safe container is if the container don't respawn items over time, it is a safe container (although this will take some time).