Building a "sympathetic" addition doesn't necessarily mean re-creating history, nor does it mean spending more for custom components. Success is all about appropriateness: in size, proportions, design, and materials.
A compatible addition starts with massing and proportion. That's because the rules are similar for, say, a cubic Italian Villa built in 1868 and a cubic American Foursquare built in 1923—not because of their style or era, but because of the shape of the house.
This site offers easy-to-understand guidelines and, best of all, a series of real-work designs along with interior views. Everything is interactive so you can go where you want and find what you need.
Use these examples in discussions with your architect or builder.
They are not meant as "blueprints," since every site and house is different. Enjoy the tour!