So, why not take the opposite approach?
Make your addition look absolutely nothing like your 18th century farmhouse. Make your addition look like a wooden cube tacked onto your home. But, and this is where it works, maintain the same modest, streamlined, minimalist interiors in the new wing and the old wing.
The renovation, done by Messana O’Rorke, of this Eighteenth Century homestead in an apple orchard in Columbia Country, New York. Each building stands alone yet interconnected.