Subdividing R-1 vs R-2, VA (Fairfax County)

[John Vandivier]

My understanding is R-X designation indicates residential where X is the number of dwelling units per acre allowed.

I fully understand why a second dwelling unit could not be created on this R-1 zoned lot of about .6 acre:

4420 Village Dr, Fairfax, VA 22030

I do not understand why a second dwelling cannot be created on this R-2 zoned lot of about .6 acre:

8909 Bordeaux St, Alexandria, VA 22309

That the second property is not sub-divisible was mentioned to me by a realtor, not the county. I haven't heard back from county's engineer of the day. Looking for info in the meantime.