The above materials requirement should read: 4 stacks of wood, 5.5 stacks of rock, 0.5 stack of metal.
Do different materials add different amounts to the total need, for instance you would need way less steel than cuprum as in MO1?
Each structure type has a fixed HP so material type doesn't impact HP.
In the live stream demonstration prior to patch release, certain materials did behave like you describe. Steel for example gave a 50% bonus meaning using 1,000 actually gave you credit for 1,500. However.. this did not materialise in the patch and all material types were treated as equal.
It is unclear if the bonus system will eventually come in a later patch or at go-live, but it has been demonstrated so it could happen.