In the case of old office buildings in Paris, they usually rebuilt the whole building while keeping the facade.
The floors added are usually a part with a very contemporary architecture.
Two new floors
Before :
Rue de la Boetie, 8e
After :
Rue de la Boetie, 8e
A new glazed roof
Before :
Boulevard Haussmann, 8e
After:
Boulevard Haussmann, 8e
Two new floors. It seems that the two buildings on the left have also been heightened but in the past.
Before :
Boulevard Haussmann, 8e
After:
Boulevard Haussmann, 8e
Here a post office buildings, as the post doesn't need all the space anymore.
They transformed most of the building into offices and added floors.
Before :
Rue de Prony, 17e
After :
Rue de Prony, 17e
Before :
Boulevard Diderot, 12e
After :
Boulevard Diderot, 12e