Thanks Pierre! That's correct.
When you have the player mat that allows you to move and build in the same turn (I think it was the engineering mat), as I recently found out by chance, you actually could build 2 structures in a single turn.
There is at least one encounter card which allows you to build a structure. This happens during the top row action.
Then, if you have the resources, you could build another structure with the build action.
So, technically speaking, my answer was not entirely true.