I have no idea why is it grainy on the left side. But on the right side it is ok. I added additional lights on the left side but the problem is still there. Any tips? I am struggling about lighting the rooms.

I have no idea why is it grainy on the left side. But on the right side it is ok. I added additional lights on the left side but the problem is still there. Any tips? I am struggling about lighting the rooms.

Artifatcs are caused by overlapping faces.
On the right side you have this situation (correct):

while on the left side (overlapped/dubled geometry):

You can clearly see the problem also in Solid view display mode.

As faces shares the same location, camera rays can't tell which one is visible from camera, so shows a bit of one and a bit of the other.
You have to create a gap between faces
your green wall's face is overlaping the other wall (behind it). move it a tiny bit away.
(i also suggest you to make your roof a bit less shiny with a lower specular intensity and hardness)