For shooting home interiors, the wider the better. I don't know Canon lenses specifically, but the Tokina 12-24 should be a good choice if you can find it - it's pretty new. It looks like a copy of the Nikon that I have used, which is really good. I've also heard good things about Tamron's 14mm prime but that would be around $900.

The problem with super-wide angle lenses (zooms especially) is barrel distortion, which will make straight lines at the edges of the frame look rounded. This is a lot more noticable in architecture than nature photographs. The better the lens, the less the distortion. Photoshop can fix this too, but life will be easier with fewer corrections.