"a couple of feet" may be enough to reduce your acreage below the point where you are allowed to do certain things. There may also be limitations on how many buildings can be present on a given lot size.

(And it can be very location specific. We're on the border between a "residential agricultural" zone and a "conservation" zone, in the residential zone. Our lot sizes can be as small as 2 acres. In the conservation zone, it is 4 acres but you can only build one a minimum of 10 acres.)