How to add a faction-specific unique building to a city?

[Bogdanov89]

The new Von Carstein vampire faction can make a new unique building (Mausoleum of the Mad Count) in Castle Drakenhof.

I want to make a mod that will add that new Von Carstein unique building as a build-able option for Castle Tempelhof once it is conquered by the Von Carstein.

The purpose of the mod is to allow two coop vampire players to enjoy the coop campaign to its fullest by allowing Von Carstein to make their unique building in a different city - one that is not already owned by the other player at the start of the game.

If someone can please explain to me how can i do this?
I have some limited experience in editing already created mods, but i have never made a whole new mod nor do i know what file needs to be edited to do what i explained above.

[Bogdanov89]

This mod solves the problem i mentioned:

http://steamcommunity.com/sharedfile.../?id=878483766

Dont forget to thank the author, he swiftly and skillfully made it.