Hi Rik, creating a new surface by displacement along surface normals (as opposed to a single overall direction like Extrude) is done with the Construct > Offset command.

You can also use Construct > Offset > Shell which will also construct side walls.

If your surface is kind of wiggly though an offset can be problematic since wiggles will get magnified.

If you could post a .3dm file with your surface in it, that could help to give other advice.

- Michael