You may wish to change your base layer to aerial imagery to see the area that needs to be subtracted,
Locate your hard standing area (or similar) and select your Land Parcel. Click on Add feature, expand Common and select Rectangle.
Click on your starting point and then your second and drag out the rectangle – single click to finish.
Now click on Finish drawing.
Select your newly drawn rectangle and click on the Edit tool. Toggle on Subtract and click OK.
The feature has now been subtracted from the Land Parcel Area.