Here's a hilarious story out of Germany - A landlord was caught sunbathing in the nude by his tenants, but when they decided to withhold rent in protest, the court ruled against them.
In an upmarket residential district of Frankfurt, a human resources company was renting an office space in a building owned by the naked sunbather in question. The company took issue with the man's nude sunbathing and tried to reduce their rental payments. The landlord responded by suing the company, and the court wasn't having it. They found that the tenants' use of the property was not affected by the landlord's choice of attire (or lack thereof). They also couldn't find any "inadmissible, deliberately improper effect on the property."
So... I guess as long as your landlord isn't causing any harm to your living space, they can do as they please, even if that means getting tan in the buff. Just be sure to keep your blinds closed if you don't want to see anything too revealing!