The second part of this item is fairly controversial. It is tempting to add on in a way that is indistinguishable from the original house. New construction should be a record of its own time, yet sympathetic in scale and proportion. If you do mimic the style of the old house, let the construction techniques reveal its origins. New additions and adjacent or related new construction will be undertaken in such a manner that, if removed in the future, the essential form and integrity of the historic property and its environment would be unimpaired.
Low impact is the name of the game.
Remember, old friends are worth keeping.