My P68 seems to handle it just fine right now but that might be a different story if my wife gets the addition to the house she has been planning!
Several words to the wise.
Quad pane low e windows, plenty of air leak sealing, insulation, insulation, and more insulation. No additional outside doors.
And perhaps some more insulation in the part of the house you already have.
Oh one more thing I don't think I mentioned it before insulation (what can I say I'm old and have CRS )