There's been a lot of talk invested in the last few years in trying to define what is an historically sensitive addition. The National Park Service has published guidelines, which, in short, recommend preserving historic features and materials in order to preserve a building's historic character. That's the goal.
The Park Service also suggests, in a general way, a means of accomplishing that. The recommendation is that any addition to an historic structure be designed in such a way that it look enough different from the original structure that a visual distinction be apparent to the casual observer In short, respect the old building but don't try to fool anyone that what you've added is old.
There have been a number of strategies devised over the years that aim to accomplish this, and I'll discuss those shortly. But first there's a question to be considered: Although the visual distinction notion has been widely accepted, is it always appropriate? In a word, no. I agree it's a good first assumption but in some cases, an architectural solution will emerge that closely mimics the original and looks just right. Working on older houses requires nothing if not flexibility.
The possibility of not obeying the Park Service dictum raises another impor-tant philosophical issue: Is it somehow dishonest to add a new-old structure that isn't distinguishable as being different from the original? Is that playing fast and loose with history?
Some would say, Yes, absolutely. I'd say, Maybe, it depends.
For me, it's case-by-case. It comes down to whether or not we identify a given dwelling as an historic house. No, I would never recommend that their caretakers put an addition on Monticello, Mount Vernon, or any major architectural monument. On the other hand, the definition of historic house has broadened greatly in recent years. You ask Foursquare and Bungalow owners whether their old houses are historic, and lots of them will tell you from the bottom of their hearts that, surely, yes indeed they live in historic houses. And I'm not going to tell an enthusiastic wave of volunteer preservationists they're wrong.
So let's look at some strategies.
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 sepa-rate the addition from the house with an even smaller hyphen or connecting struc-ture 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 circum-stances, but the nature of an individual structure must be factored into deciding what to do.
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.