Brittany was a sovereign duchy until the treaty of Verger.Before then,it was never conquered by the Franks nor the French.Normandy whereas was originally a fief granted by the West Frankish King to Rollo.
Now I understand what you meant and I can explain precisely what your mistakes are .
You are mixing up 4 different things :
- being independant/sovereign (Britanny ceased being independant/sovereign around 940, becoming a duchy inside the kingdom of Francia occidentalis),
- being a duchy not incorporated in the royal domain and held by an autonomous dynasty and ruled by this autonomous dynasty (which was the case for Britanny from 936/939 to 1491),
- being a duchy not incorporated in the royal domain but personnally held by the king or his wife or his son, (which was the cas for Britanny from 1491 to 1547),
- being a part of the royal domain (which was the case from 1547 on).
The other mistake you're making is the analogy between England and France.
These 2 kingdoms were not established the same way and their respective ruling dynasties did not get the throne the same way.
In England, William the conqueror conquered an existing kingdom of England. He did upset most of the local nobility and establish his own nobility, which made possible for him to build the most precocious and most centralized State that existed in western Europe.
There never was something like the domesday book in the kingdom of France.
France did not exist when the small frankish dynasty of the Merovingians appeared and became a significant actor. The Merovingian dynasty just happened to rule a part of the rather small germanic tribe called the franks. And they were able to take a part of what had been the gallic provinces of the crumbling roman empire.
They did not conquer it.
They just happened to be federate soldiers at the service of the dying west roman empire.
They were just the remaining soliders of the roman empire in their part of Gaul. And they made alliance with the local gallo-roman aristocrats. And they fought and conquered other rivals that had done the same thing in other parts of Gaul, as well as in parts of non roman territories, on the east bank of the Rhine.
"King" was just the title that qualified the power of the Merovingian dynast on the frankish tribes.
The power exerted by the king of the franks over the territories he controlled was first a de facto power (going through the rather classic ways of the local roman administration system) that gained some formal recognition by the sole remaining roman emperor (the one standing in Constantinople) through titles such as Patricius.
It took a long time of ruling on these territories by this frank dynasty that the word "franks" began designating other people than just the members of this tribe and that Francia became the name of a territory.
There are other subtle details about the evolution of the geographic meaning of the name "Francia".