The Prerequisite System

The Prerequisite System controls the conditions required to build a certain object. Those conditions include:

Can this house own this object?

Checks if the house is listed in Template:TTL.

Is this house not forbidden from owning this object?

Checks if the house is not listed in Template:TTL.

Is this house allowed to own this object?

Checks if the house is listed in Template:TTL.

Does this house own all the prerequisite structures?

Checks if the house owns :

all structures listed in Template:TTL

or

at least one structure listed in Template:TTL.

If all the conditions are met, the house is allowed to build that object. Accompanied by the EVA_NewConstructionOptions sound.

Then an additional check takes place:

Does the house own the Template:TTL of the appropriate type?

Checks if the house has at least one structure with Template:TTL matching the object's type.

Note: For VehicleTypes, it also checks if said Factory has the same Template:TTL setting as the object in question. If it doesn't, the object is removed from the Units tab, and, if that tab is open, the view jumps to the top of it. This is known as the New Construction Options Bug.