The Patio can be accessed either from the Upper Gallery of the Hall of Heroes (the door is located just above the main entrance) or from the Safehouse. Note that you can't access the Safehouse from the Patio until you got the Key from Auryen because the doors are key-locked.
At first the Patio is empty and provides just a view over Solitude and its harbor. After buying the two upgrades from Bits and Pieces in Solitude (Safehouse Patio Seating Receipt and Safehouse Patio Greenhouse Receipt), an awning with table and chairs as well as a greenhouse is added.
The awning is merely only a visual upgrade where you can sit on the patio and relax.