The topology of a block can be edited using the Add surface tool which allows you to create a block surface by drawing a perimeter on an existing block surface and drag it inwards or outwards along an axis, normal to the surface. It is a special case of the Drag block face tool which is used to drag a whole face. Add surface allows you to drag a portion of a block surface creating a whole new topology.
To use the Add surface command, first go to the building level, select the block that you want to modify and click on the Add surface tool. Then move the mouse cursor to the surface that you want to edit and draw the perimeter of the surface you wish to create. Once the new surface perimeter has been defined use the mouse to start dragging it inwards or outwards. You can key in an exact distance to be used in the drag process. When you have the block geometry you need, left mouse click to confirm.