I run Appraisals Guaranteed, a real estate appraisal firm in Northern Virginia. Often I'm asked what is the coverage area. I'm posting the list here. Contact me at rickphillipsjr [at] gmail [dot] com or head over to www.appraisalsguaranteed.com

I'm based out of Alexandria, Virginia (Fairfax County); so, obviously, I hold more expertise there. On the flip side, I believe you have to take the far-away orders to justify receiving the close ones.
That, and, well, I enjoy driving out to Madison County every now and again. What it really comes down to is the amount of information I have. If I had good info, I could appraise a house on Mars!
In addition to Washington, DC, here is the county coverage list:
Alexandria City
Arlington
Fairfax
Fairfax City
Falls Church City
Prince William
Loudoun
Stafford
Spotsylvania
Fauquier
Fredericksburg City
King George
Culpeper
Mathews
Warren
Caroline
Middlesex
Madison
Hanover
Contact Rick Phillips, Certified Residential Appraiser, on gmail: rickphillipsjr (and then the @ sign) and gmail.com.
For the full wesite (with telephone numbers, etc.): www.appraisalsguaranteed.com