Although I could be wrong, from what you stated it does not appear that your community has an active Homeowners' Association. Also to avoid the fiasco of a previous time when the poster here said that I do not know what I am talking about because he neglected to tell us he lives in Loudon County and they have different laws (I was quoting Fairfax County regulations), I am guessing the property is in FFX County, so lets skip to the next step.
Building permits and inspections were required when the previous owner did the conversion. Check to see if this was done.
Second, check with Fairfax County on zoning for the specifics for your area. The next step is to get an approximate count of how many people are living in the house. I am not an expert, but I do know there are specific rules for the number of people living in a house (max 4 unrelated and other stipulations). Also some rules regarding bedrooms and apartments including bedrooms must have ventilation and a window. Apartments must have an entrance/exit to the outside. In other words, if there was a fire in the house, could the basement apartment people "get out" without going through the house? Sub basement apartments without a separate entrance/exit are frowned upon. For specifices look at
http://www.fairfaxcounty.gov/hd/hdpdf/healthyhomes.pdf . Most, if not all, of this is Commonwealth VA law although other jurisdictions may have additional requirements, so do not slam me on this publication if you live in another county.
these and other matters (including overcrowding) may be reported to the County Health Department.
Count how many people are living in the house. For good measure, get the license plate number of all vehicles associated with the house. Pay particularly close attention to out of state plates and anonymously report them to the county as tax evaders
http://www.fairfaxcounty.gov/dta/taxevaders/addRecord.asp . This will make the tenants lives miserable!!!!! Also, look for cross plating - two cars having the same plate on the back of the vehicle and no front plate. If you see this, call the County Police 703-691-2131 and ask to have the vehicles ticketed. The police will come, ticket both vehicles and remove the plates. This is a major pain in the arse for the tenants.
If they are violating rules and you decide to report them you must do your homework and it is a long and frustrating project - and the tenants know this. It is worth it to bring the neighborhood back to normalcy. Good luck