For an example I just made a recipe using 8 lbs of extract, 5 gallon batch size, and a 4 gallon boil. When you click "Brew It", then click "Water Requirements", there's a table that lists the following: Target batch size: 5 gallons; Boil water added to kettle: 3.32 gallons; Volume increase from sugar/extracts (early additions): .68 gallons; Starting boil volume 4 gallons.
So yes, the extract counts towards the boil volume.
Edit: I just remembered, you don't have to click "Brew it" for that, you can click Edit/Recipe tools/Quick Water Requirements.