There is no cut-off, government buildings have 2 separate trees, one tree for major settlements, and one tree for minor ones. Visual aids tend to help, so:
Here is a picture of \building_levels (I used PFM 1.11b as it can sort columns).
Government buildings with "minor_" in front of their name are in the "government_minor category", meaning only 1-Slot cities (Brussels, Alsace-Lorraine, Stuttgart, and Cologne for example) may build them.
The other government buildings of the "government" category are built in 4 and 5 Slot Cities (London, Paris, and Berlin for example).