When many of the current Paris buildings were constructed (17th, 18th, 19th centuries), the first floor (above the ground floor, originally used for the concierge) were the apartments of the wealthiest people. The windows on that floor were usually multi-paned, more elegant, and taller, which denoted higher ceilings, a mark of distinction to the wealthy. The exterior decoration on these floors—balconies and railings—was also fancier. This made the first floor apartments the most desirable. Since buildings did not have elevators at that time, all wealthy people could at least walk up one flight of stairs.
On each floor above the first, the windows became gradually smaller (apartments with lower ceilings); and the exterior decoration and/or grillwork less fancy—the same with the balconies, if any. This, in turn, related to the social status of the inhabitants who lived in the apartments on those floors. 
The top floor, the floor that housed the servants (some below a mansard roof), often had dormers with tiny one-paned windows. Since the servants had the lowest rank on the social scale, they were the ones to climb all those stairs. Those rooms usually had ceilings too low for a person to stand fully erect; but by the time the servants made it to their rooms, they were too tired to do anything but go to bed anyway! 
As these older buildings are renovated and elevators are added, the wealthier Parisians are moving to the top floors and away from the noise and dirt of the streets. Since the building concierges have all but disappeared (replaced with code-protected doors), the ground level is rented by retailers or offices. Builders add elegant penthouses to the top floors and the apartments cost more the higher in the building you live.
The top floor, the floor that housed the servants (some below a mansard roof), often had dormers with tiny one-paned windows. Since the servants had the lowest rank on the social scale, they were the ones to climb all those stairs. Those rooms usually had ceilings too low for a person to stand fully erect; but by the time the servants made it to their rooms, they were too tired to do anything but go to bed anyway! As these older buildings are renovated and elevators are added, the wealthier Parisians are moving to the top floors and away from the noise and dirt of the streets. Since the building concierges have all but disappeared (replaced with code-protected doors), the ground level is rented by retailers or offices. Builders add elegant penthouses to the top floors and the apartments cost more the higher in the building you live.
