The closer the better but at least six inches.French drains are trenches filled with gravel or rocks that allow water to flow through them into a drainage system. They are often used in landscaping to prevent soil erosion and flooding. A foundation should be at least six inches away from a French drain to avoid any damage that may be caused by the water flowing through the drain.

It depends on the type of French drain. A trench French drain should be about 6 inches below the surface and a perforated pipe French drain should be about 12-18 inches below the surface.