Dear Hyder. There is no reason you cannot use $/sf to make comparisons between houses. This must be done carefully, however, since SF records are often not accurate, and must be done between 'similar' house types, (be sure to compare apples to apples only!). A picture then emerges to tell you if particular houses that interest you are in the right "ball park". For these reasons a $/SF number cannot be given to an entire zip code, but is definitely used when professionals are helping you make an offer on a specific property.
Having said that, within zip code area 20016 there are quite 'consistent' neighborhoods, with similarly sized and aged houses, (Kent/Palisades, Spring Valley, Cleveland Park, AU), so doing this is a good jumping-off point in valuing a properties, adding or subtracting for special features as you go along. This is easiest when considering condos or coop apartments, (for the obvious reasons) and I believe the DC average $/SF for a condo is still about $500. Houses throughout the District range between $350/sf to about $1,000/sf, (for VERY special places, anything above or below such numbers gives reason for thought.
Hope this helps! And since I live, work and specialize in the 20016 areaa, if you'd like more detailed information, please don't hesitate calling: 301-801-2970 or visiting me at: http://www.cbmove.com.