From Valve Developer Community

The Make Hollow command takes a selected object and generates walls out the shape, making it a hollow room.

Warning: This tool is actually a shortcut for duplicating the brush, scaling it, and carving, which can result in poor brushwork!

The Make Hollow dialog will ask you to supply a value for wall thickness. Positive numbers will create the walls inward, and negative numbers will hollow outward. Each of walls that are created as separate solid objects, and are grouped together as one. You can Ungroup them to select and edit them individually. Several objects can be selected to Make Hollow at once, and the command will make separate hollow groups out of each of them.

To make an object hollow: