Landlords must give advance notice of an apartment inspection. Tenants can refuse not only if landlords come over spontaneously and without a good reason, but also if the inspection takes place unreasonably frequently.
If you view the apartment in their absence, you are committing trespassing. This can be punished with a prison sentence of one year or a fine. If the apartment is to be rented or sold, all rooms must be accessible. It doesn't necessarily have to be tidy, but: "A room shouldn't be cluttered with furniture"