Your problem is your lighting. Since your only sources of lighting are the windows, Blender is having to compute a lot of bounces to get light the room, which results in a super grainy render. The first thing to do is to add light portals. These will direct Blender toward the windows for light.
- Add an Area lamp. Position and rotate it directly behind your first window, pointing in, and size it just bigger than them. Change the shape to rectangle, and size it to fit the window. Check the "Portal" box.
- Duplicate it and move it to the other windows. (For convenience, since they are all the same size, you can do a linked duplicate
The above should help you get more light from your sun. However, you still don't have enough light overall in your room. I would add some other lamps either on the ceiling, behind your camera, or anywhere there is actually light (since you are replicating your own living room).
Also, since you are using an HDRI, enable "Multiple Importance" under the "Settings" panel in the "World" tab.
One last thing you can try is to increase the Max Bounces underneath the "Light Paths" tab. This will force Blender to calculate each light ray further, but this is very computationally intensive, and will do relatively little for you compared to other options.
EDIT: I just looked at the number of samples you are using, and those are crazy low. Crank those up! 50 at least!