9 out of 10 times the 16-35mm will do what you want. 16mm is pretty wide on a 6D. If you find yourself needing ultra wide, Canon has a 8-15mm L lens that lets you take a photo of your feet while holding the camera at eye level and looking straight out!
This is 15mm inside the CSO Performing Arts Center.
The 16-35 is a good indoor lens for real-estate. The TS-E 17mm and TS-E 24mm would be even better, but those are special purpose lenes.
When a lens is pointed upward or downward (relative to the horizon) vertical structures in the image (doorways, windows, etc.) will pinch inward or outward. That means doors take on a trapezoidal shape instead of a rectangular shape. To keep that from happening, the lens needs to be "level" to the horizon line (to the floor of the room). A tripod would REALLY help to let you get the shot framed while keeping the camera level so you don't get trapezoidal distortions.
When you shoot real-estate, you'll find a lot of these trapezoidal (keystone) distortions. To minimize the problem, you'll want to be mindful of the camera angle.
But what happens when you find that the room looks better when shot from a certain position... with the camera off to one side... or a high angle or low angle, etc.? But those angles create the distortions. If you move the camera to a position where you don't get the distortions, it's no longer the room perspective that you want. This is a problem that "tilt shift" lenses can solve.
The TS-E lenses (tilt-shift) are specialty lenses. They have an aticulated lens that can shift along one axis and tilt along another axis, and each of those two axes can be rotated so it can do this in any direction you need. This provides the ability to shift perspective and also alter the plane of focus. The lenses are ideal for both architecture and landscape photography. BUT they are manual focus only (no such thing as an auto-focus tilt-shift lens... from any manufacturer.) This means the lenses can be used to make sure that verticals actually remain vertical.
These two shots were taken with the same camera, same lens, and same position (camera was on a tripod). I am using the Canon TS-E 24mm f/3.5L tilt-shift lens. For the first image, the tilt and shift axis are all zero'd out (no tilt, no shift... in other words it's working like a 24mm prime lens with no funny-business going on.)
In the second shot, I adjust the "shift" axis to correct for the perspective distortion of the building.
Notice that in the first shot, the building appears to be "leaning" back. In the second shot it appears upright. That's what the perspective correction provides. This can be done in Photoshop, but it requires an extra-wide shot (because you're going to lose a bit of the sides of the image after you perform a "keystone" correction and then crop the image back to a rectangular shape. (BTW, the brain expects some perspective narrowing as distances get farther. If the building is made to be perfectly parallel top to bottom, the brain may perceive an optical illusion that it seems to get "wider" and not remain parallel. So I'll use the grid-lines in the camera to dial in a "perfect" parallel correction.... and then back off the correction just a tiny bit to keep the brain happy. That means the image is technically just a tiny bit narrower at the top then at the bottom.)
The "tilt" axis allows you to alter the plane of focus based on something called the Scheimphlug Principle. http://en.wikipedia.org/wiki/Scheimpflug_principle It's ideal for landscape because the plane of focus can be laid into the plane of the landscape itself.
Again... tilt-shift lenses take a bit of learning (especially the "tilt" axis.) They can be VERY frustrating until you learn to use them because they aren't going to help you get the shot... you just have to take the time, do the reading, do lots of practice, etc. I actually took the time to learn the math, so that I could physically measure the distances and angles and determine the ideal tilt-angle, then tested the lens to see if my math was right (it was). I don't know any photographers who get out measuring tapes and calculators when using the lens, but it helped it "click" for me so that I finally understood what the lens was doing.
It'll be easier to learn with a nice wide-angle lens and the 16-35mm is an excelleng choice for most real-estate and landscape needs. I only mention the tilt-shift because you'll quickly notice the perspective distortion issue and will be wondering how other photographers solve that problem (and don't forget it can also be solved via Photoshop.)
