Yes, you can define which floor is the ground level and even easily change the order of existing floors - in the layout screen, tap the floors button, then tap the edit button. You can now drag the floors up and down to reorder them, add new floors and delete/rename existing ones. Make sure you are in the layout screen when attempting to reorder floors or otherwise the edit button will not show. The layout screen is the screen where you can drag rooms around (on the top toolbar, at the left,  there is a 3-button trio - the layout button is the leftmost button with the white rectangles icon on it).