Buildings are defined in this file. New province buildings are possible, but require modding the interface for the game not to crash when you click on a province. Province buildings can generally have any modifier effect that an event modifier can possess, but there is an extremely significant limitation: for most of them, the building may only have one (the last defined one being the one that counts). Another issue is that although the modifier effects do work for the province buildings, and they do show up properly in the tooltips of the province itself, they do not show up in other tooltips where they should. The AI will not build new province buildings by itself, so you will need to create special events or decisions for it to construct the new buildings.
Cost
The "cost" field for buildings defines an extra money cost that will be applied on top of the money cost calculated based on the goods the building costs. Interestingly, this extra cost scales with level. In the base game, only the naval base makes use of this field.
not_if_x_exists
The "not_if_x_exists" field determines which other buildings impede this one from being constructed. It has been confirmed to work for province buildings, but it is unknown if it works for factories.
Syntax:
not_if_x_exists = { n }where n is the name of the province building to be incompatible with this one
Note that although this works correctly, the interface tooltip will have an issue. It will display the name of the building to be constructed itself, rather than the one blocking it. It is possible to alter the localisation tag (BUILDING_BLOCKED_BY) to refer to a particular building, however.
prerequisites
The "prerequisites" field defines which other buildings any necessary in the province before this one can be constructed. The field has been tested and confirmed to work for province buildings; whether it works for factories is unknown.
Syntax:
prerequisites = { n }where n is the name of the province building to be required
Note that although the requirement works correctly, the displayed text does not. For example, if you create a university building and make it require a school, the displayed text will nevertheless be "Must have University", rather than "Must have School". No way is currently known to fix this, as in the relevant localisation tag (BUILDING_MUST_HAVE), the "$OTHER$" is interpreted by the game as referring to the name of the building to be constructed itself, rather than the prerequisite. It is possible to change that localisation tag to directly refer to the name of a particular building, however.