Yamataca

It might be too late for my answer, but maybe this helps someone else struggling with dark rooms. Took some time to find this out trough long experiments..that 'Attenuation distance' is to fault. Less number it has, lighter the walls become inside building. Here are my example for Ambient Occlusion bake. Not using any lights, lamps etc. House has roof on.

For me this happened because i saved my mesh in folder with no englesh letters: like ā, ī, ķ and etc. Changing folder name fixed issue.