Someone has already recommended Loudoun Co. and you'd probably need to go to at least Purcellville or further west on Rt. 7. I've not lived in Loudoun in many years but visit there and it's so grown up now that I hardly recognize it. The Lovettsville area in Loudoun might work for you, as well.
Other options would be northern Prince William Co. but you must live within the "Domestic Fowl Overlay" on the county map and have at least 2 acres to be able to keep chickens. Even with 2 acres you're limited to 20 chickens. The more acreage = the more chickens you can have. The great majority of the northern section of PWC is included in the overlay but much of the Gainesville/Haymarket area has built up so that it's difficult to find a 2+ acre plot. Most of it is really large tracts or subdivisions. I'm currently in PWC. There are commuter lines to DC including the VRE train and a bus route that leaves several times each morning from numerous stops in the region, including Manassas and Gainesville. Of course, there's always the option to drive to Vienna to catch the Metro Rail.
I'm thinking your best bet for agricultural activities would be Fauquier County. Specifically out Rt. 28 heading towards the Bealeton area. It goes through communities such as Nokesville, Catlett, Midland. Another option would be the more northern Fauquier area near Marshall/The Plains. Prices in Fauquier are much better than Loudoun/PWC for most everything. I live in PWC and my daughter is in Fauquier. We always fill up our truck with gas in Fauquier when we go to visit her as the price of gas is significantly lower. Larger tracts of property are more common and Fauquier considers itself an agricultural county. The commute to DC would be more difficult as you'd have to drive into at least Gainesville or Manassas to participate in any public transportation. The Rt. 50 area of Fauquier is a bit more expensive to live in as it's considered "horse country". Fauquier is near-and-dear to me as I was raised there.
In any case, you'd want to check with the individual counties on laws as I'm sure they're different between communities within the county. Welcome back to NoVa and Good Luck!
Mtn Laurel... Wow... I am VERY appreciative of your taking the time to write all this! Thank you so very much. You have given me a great deal to work with now. Thank you!