Quantico Creek - sediment clogging a former harbor
There are four towns within the boundaries of Prince William County - Occoquan, Dumfries, and Quantico near the Potomac River, and Haymarket in the western end. Oldest town in Prince William is Dumfries.
Youngest town is Quantico, which was intended to be a resort center when founded a century ago. When that did not succeed, the Quantico Company sought to make it into an industrial center for building ships. In 1917, as the US prepared for entering World War I, the company was happy to sell its property to the Marine Corps.
Today, the town of Quantico is surrounded by Marine Corps Base Quantico (the "Crossroads of the Marine Corps") on three sides and bordered by the Potomac River on the fourth. To enter the town of Quantico, you must pass through base security, arrive by boat... or take the train. Though the Marine Corps is moving 6,000 people to the base after Base Closure and Realignment Commission (BRAC) decisions), and defense contractors try to locate their offices within 15 minutes of a military base... few Class A office buildings may be built within the boundaries of the town, because access might be limited.
Instead, defense contractors may locate in Dumfries or in Stafford County. Most properties in Dumfries are partially-developed, though the buildings may be relatively low-value. Existing development complicates proposals to build new office buildings, and getting approvals for changing land uses could involve delay. Redevelopment along Route 1 involves more risk of delay than building on "green fields" in Stafford.
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Lake Ridge, Gainesville, Nokesville, Independent Hill, and other named places in Prince William County are not incorporated. Some very large communities in Northern Virginia are not incorporated, and are governed by county officials (and home owner associations, for modern planned developments). Tysons Corner and Reston are place names in Fairfax County, but are not towns (like Vienna, Herndon, and Clifton) or cities (like Fairfax City and Falls Curch). There is no official "mayor" of Reston, no "town council" of Tysons. The elected local officials for those area are the members of the Fairfax Board of County Supervisors.
NOTE: There are advocates for incorporating Reston. The Reston Citizens Association supports converting a community of 60,000 people into a town, empowering Reston residents to reject rezonings that Fairfax County is approving to add more housing. Robert E. Simon (i.e. "R.E.S.town") intended to create a town at Reston, but Fairfax County has always opposed balkanizing the county into multiple small towns.
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Also, home owner associations (HOA's) are not official government agencies. Dale City was a planned development (road names follow the alphabet, starting with Ashdale, Birchdale...), but all the HOA's are private organizations.
If you purchase a house within a subdivision with an HOA, your deed to the property will state that you are required to be a member of the HOA. The HOA can force you to comply with HOA regulations, such as restrictions on signs in the front yard or the colors you can use to paint your house. The HOA can put a lien on your house, if necessary, to collect HOA dues. (A lien forces you to pay the debt before you can collect any money from the sale of your house.)
However, HOA's are not government agencies. They are private organizations. Homeowners sign a contract (in the deed) when they buy a house in that subdivision.
In addition, some neighborhoods may have a voluntary neighborhood organization. It can be difficult to tell from the name - for example, the "Burke Centre Conservancy" in Fairfax County is actually the HOA for a 1,700-acre Planned Residential Community south of the Fairfax campus of GMU. That HOA elects seven members to a board each year to oversee the regulations affecting five Burke Centre neighborhoods, but the HOA is a private corporation that operates according to its bylaws rather than a government agency.