I find that a 20-28mm lens (full frame) or a 14-20 (cropped sensor) to be the most useful focal lengths. I am a real estate agent that was a professional photographer for 12 years. I shoot primarily my own listings, listing for some of my agents and the occasional paid shoot for various other agents.
I personally own a 10-20mm Sigma on a Nikon D7000. I have found 10-14mm provides to much "perspective distortion" for real estate photography.
As a real estate agent who shoots real estate photography, I have to listen to the buyers. There is definitely a balance for getting the feel of the room with out making it look way larger then it is. Buyers get extremely annoyed when they walk into a home and it does not feel like the photos.
You want to use the widest angle lens possible with out too much perspective distortion. The objective to maintain is you want to get a fairly accurate rendition of a room so you may attract the right buyer to the property. If you inaccurately portray a home either thru real estate photography or its description buyers get pi$#!d! and will walk out angry and not seriously consider the home.
Finding that balance is an acquired skill. When I first got the 10-20mm everything was shot at 10mm... so great, I could get everything in.... :( not so great I heard a lot of complaints. As I work on my skills, I find I rarely shoot below 14mm on a 1.5 crop sensor and try to get closer to 17-18mm.
As far as lighting goes. My two typical scenarios are an available light shot blended with a multiple flash shot or an Exposure Fusion shot (not to be confused with HDR) with up to 7 brackets blended with a flash shot. I use up to 4 flashes.
This was shot at 14 mm you can still see perspective distortion especially in the lamp. I wanted to show the fireplace and the window with this particular shot. Though overall this shot did not really make the area look much larger than it is. This was 7 exp fusion with a 3 flash shot blend with a separate exposure for the scene out the window.