The mud wasp nests I usually find inside or outside our house look like this one on a windowsill:
Sometimes they look like this one on the outside wall of our house:
The picture below shows a wasp nest that had been built between a box on top of a built-in cupboard and the ceiling, which probably accounts for the squat shape. You can see ceiling paint on the top of it:
The back of the nest looks like this – note what is left of a spider in one of the cells:
This nest was probably built by a Mud Dauber Wasp (Sceliphron spirifex) which commonly builds multi-celled mud nests attached to walls or even tree trunks. Each cell is provisioned with a spider that has been paralysed in order to provide food for the emerging larvae.
Here is a not very clear picture of a wasp spotted in the garden taking a spider to its nest.