ESO Free Looting Areas

Rustwall Estate in West Weald - do quest to unlock catacombs, area resets every time you step into catacombs so you can keep looting Rustwall to farm recipes.

Necrom Necropolis in Necrom - Telvanni Peninsula - may require a quest to unlock, has enemies inside, but also a lot of urns and other containers to free loot. Necrom in general is also a great place for thieving if you have the right build.

Councilor Jirich's Manner in Leyawiin - requires quest to open but then can access and freely loot

Adding good area to thieve in the game in order to farm furnishing plans:

Fargrave -The Bearer's Rest. Enter through eastern door, run upstairs, loot the cabinets and backpacks, and run back down and leave. When you go back in the cabinets and backpacks will be reset.