I've talked to several Real Estate Photographers recently in the process of purchasing new DSLRs and noticed that there is a misunderstanding about what lenses are appropriate for real estate photography on the new DSLRs.
First of all there are two types of DSLRs:
What does this have to do with wide-angle lenses? Everything! When you mount a lens, say for example a 18-55mm, on a full frame DSLR, it's a 18-55mm lens just like you expect. But when you mount the 18-55mm lens on a APS DSLR the lens doesn't act like a 18-55mm it acts like a 28.8-88mm lens! This is because the smaller sensor size of APS cameras have the effect of multiplying the focal length by a focal length multiplier (1.5 for Nikon and 1.6 for Canon). The term used to refer to the the focal length after the multiplier is 35mm effective focal length.
This focal length multiplier is a big deal for real estate photographers because this means the standard kit lens that comes on most DSLRs (18-55mm) isn't optimal for real estate work. You may be able to squeak by in big rooms but as soon as things get tight you're in trouble because you won't be able get the shot because your back is against the wall.
Real estate photographers find the effective focal lengths between 16mm and 24mm to be the "sweet spot" for shooting interiors. It's best to have a zoom that covers this whole range between 16 and 24 but at a minimum you need to work at 24mm or below. This is why the Sigma 10-20mm lens (available for both Nikon and Canon) is so popular with real estate photographers, because with a 1.6 multiplier it allows you to work between 16 and 32mm effective focal length and it's an inexpensive alternative. For full frame DSLRs the Canon 17-40mm, Canon 16-35mm and Nikon 17-35mm lenses are popular choices for interiors.
What are the wide-angle alternatives for APS DSLRs? I'm only going to cover Canon and Nikon because I recommend that you stick with these two manufacturers. It will make your life easier and give you more flexibility and alternatives in the long run because 3rd party vendors provide accessories for these two brands.
Notice that I put the Canon 14mm fixed lens on the Canon list. This lens is legendary for it's lack of barrel distortion. It is pricey but as high a quality as you'll find. Zoom lenses all have varying degrees of barrel distortion that can be removed in Photoshop or other photo-editing software.
The bottom line here is that you need to pay careful attention to which lens you choose for real estate work. It maybe the most important equipment decision you make for real estate photography. It's way more important than which camera body you choose.