There should be some options in the XML to allow certain type of terrain to be constructable.

The is currently 2 problems

First: Some terrain cannot have building placed on them.

Second: The sovereign (and probably pioneer) is restricted to build a new city on certain terrain.

All this information is hard coded, so it should be placed in the xml instead.

The building terrain can be overrided by adding terrain requite to each building. Which allows me to build on cliff and beaches. An screenshot is posted at the end. But I did not find anyway to override new city construction.