For a small, simple storage building:
* Cabane (pronounced ka-ban) is the most common word and is often used for garden sheds.
* Remise (pronounced ruh-meez) is a more formal word for a storage shed.
For a large, detached building:
* Bâtiment annexe (pronounced ba-tee-mon a-neks) means "annex building" and is used for larger, more elaborate sheds.
For a specific type of shed:
* Hangar (pronounced on-gar) for a large, open-sided structure used for aircraft or other vehicles.
* Serre (pronounced ser) for a greenhouse.
Here are some examples:
* J'ai une cabane dans mon jardin. (I have a shed in my garden.)
* La remise est à côté de la maison. (The storage shed is next to the house.)
* Le nouveau bâtiment annexe sera utilisé pour stocker les outils. (The new annex building will be used to store tools.)
* L'avion est garé dans le hangar. (The airplane is parked in the hangar.)
* J'ai une petite serre dans mon jardin. (I have a small greenhouse in my garden.)
Choose the word that best fits the context you are describing.