Don't worry about diffraction for this purpose, but with a lens that wide and shooting on a tripod, you probably don't need more than f:16 anyway.
The main issue with a 20mm for interiors--if you want the image to be realistic--is that it tends to make the space look larger than it really is. Sometimes, it's your only choice, because the space is too cramped, and you don't have anywhere to stand. I try not to go wider than 24mm (with the 35mm format) for interiors. Experiment and see how it looks to you.
About lighting--are you shooting in color? If you are, mixed lighting can be a problem. You might have mixed incandescent, fluorescents of different colors, and window lighting. Then if you want to balance indoor and outdoor lighting, that's another issue. If you want to show the view out a window, for instance, you want it to be about one stop overexposed, so that it "looks like the outdoors" with the indoor scene exposed properly, but in reality, the outdoor scene will be about four stops brighter than indoors in full sun. The options are not to shoot during full sun and/or light the indoor scene and then you usually want everything to have the same color balance. Another question is whether the indoor lighting is something you actually want to show--maybe it's important to the architecture. If it isn't, then just turn it off. If you want to get fancy, you can gel everything to the same color temperature, but the easiest approach is usually to turn off the indoor lighting and light the interior with strobes that will balance the daylight from outdoors.
For an introduction to all the issues, take a look at Norman McGrath's
Photographing Buildings Inside and Out:
http://www.amazon.com/gp/product/08...de=as2&tag=davagol-20&linkId=J27Q2KIE7QJCRO6J