Sandbox mode allows you to build houses, and create jobs for other players to complete. Extra tools and items are available in sandbox mode that are not available in the career mode, and there is no limit on how much money can be spent.
In sandbox mode you can fly around the plot and go through walls for easier building, and actions can be undone ('Z' on PC).
Jobs and houses that you create in sandbox mode can be exported and uploaded to mod.io to share with other players of House Flipper 2.