How accurate are your home addition plans?

Basically how much of a field adjustment is considered normal? Difficult to get an exact measurement on an existing house...

We had an architectural draftsman come out, we gave him our concept and he drew up a permit plan set from an AutoCAD survey we had done when we moved in. All good.

However, we wanted to make a few changes to the plans (I'm a landscape architect), with his permission I am drawing up plan set into AutoCAD. Mostly it's just adding detail to the plan, roof penetrations in certain places, making sure everything will line up correctly etc.

But I realized that the survey we gave him originally included this existing stone siding in the measurement instead of the brick foundation (I assume because I'm getting different measurements on the existing house). We're essentially extending out the entire back of our house so it's pretty essential that the foundation walls line up. His drawings say 67' 6" and the existing house measures more like 67' 3".

Pretty sure at this point I'm going to adjust everything to be more accurate in the cad drawing, but my question is how much field adjustment and measurements is considered ok?

Is it better to be slightly over or under?

Thanks!