The Sensitive Addition

REDUCED SIZE AND SCALE.
One good way to think about an addition is that it should be smaller in scale and overall size than the original house. If your house is a Classic Colonial, with a facade that's two stories high and 40 feet wide, the wing you add to one side might be a story and a half and 30 feet wide.

RECESS THE ADDITION.
Another common recommendation is that the front plane of the addition be noticeably recessed back from the original structure, a visual acknowledgment of its secondary status. A variation on the same theme is to sepa­rate the addition from the house with an even smaller hyphen or connecting struc­ture that further distances the original house from what you've added. Another proven strategy is to make the addition invisible from the front—for centuries, here and around the world, important building facades have been left unchanged when necessary additions were attached to the back rather than to the front of a building.

TO MATCH OR NOT TO MATCH THE EXTERIOR FINISH.
Not everybody agrees here: One camp argues that the siding, window trim, and other detailing should be consistent with the original; another group advises subtle changes are essential, such as simplifying the trim or using shingles to contrast the original clapboards. Both approaches are, in my opinion, perfectly correct under the right circum­stances, but the nature of an individual structure must be factored into deciding what to do.