The Sensitive Addition

RESPECT THE ROOFLINES.
Different rooflines will probably draw immediate attention to the addition. A radically different roof shape (a flat roof abutting a gable roof, for example) is likely to look wrong. Adding dormers, a cupola, or other elements not on the original may also look peculiar. While you don't have to copy the cornice, eave overhang, pitch, or even the overall roof shape, a complementary configuration that echoes the original is probably the best strategy.

I recommend you take all of this reasonable advice and mull it over. Take what fits and feels right—and be forewarned that you won't please everyone. The truth is that working on an old house requires a series of judgments. You need to think like an architectural historian, a builder, a curator, and a homeowner.

Remember, too, that you're just passing through. The odds are that the house will be there generations from today... so treat it with respect.