The red building is the New Chambers which was built as an orangery for keeping the plants inside during winter. Frederick the Great changed his mind to convert it as a luxury guest house.
Behind the New Chambers, what looks like peeping over the wall, was an old mill that had been there six years before Sanssouci Palace was built.
Rumour has it that the miller complained that the palace was blocking his view, and that the king was not happy about the noise and tried to force the miller to move out.
Actually what happened was the opposite.
When the mill needed repair, the king financed the construction of a larger and more functional stone-based Dutch-style smock mill.
The Prussian kings deliberately preserved the mill over the generations as a proud symbol of the rule of law—representing the idea that even a king must respect the property rights of an ordinary citizen.