I am sure you can get in there, you will have to go to the FBO. Where they will most likely get you with a huge fee. Landing fee, handleing fee, then a fee which can be waved if you buy like 100 gallons of gas or more, which blows for a 172. Fuel at iad has got to be up above 5.50, I know someone who fueled up jet and it was almost 7 bucks a gallon.
to get into sfo its around 175 bucks to land. period. no way around it, unless you have a red cross painted on your tail.