Hopefully this might give you a few ideas?

Its a Google image search for 'Underground City', and it comes up with a mixture of real and fantasy settings.

The two extremes are what I would call 'cosy', where a house is represented by a collection of interlinked chambers stemming off from the main tunnel and totally enclosed, or 'cavernous', where you have an entire city built like a surface city, but covered over by the spectacular dome of a vast cavern.

You could go somewhere in the middle, or choose an extreme. Up to you

Don't think you will get away without roofing, though. What about bat droppings, or drippy stalactites? In a fantasy world you might also have a varied collection of large and dangerous underground beasts that might pick you out of your bed without a roof to protect you.