I'm trying to convince my sister to hire a residential architect who specializes in historic design/renovation to help her with her old farmhouse. I'm darn near close to offering to pay for his services myself.... :-)

Cut to the chase answer: As Ms. Dame suggests, find an experienced and talented architect who does remodels to existing homes.

Now that that's out of the way...you haven't furnished nearly enough information for an intelligent response.

--Why don't you furnish a floor plan and exterior photos of each façade of the existing structure;

--How about describing the sort of add-on you are considering: functions, sizes, important criteria;

--How about describing the size of the property;

--How about describing the orientation of the existing building;

--How about describing your goals and intent for the project;

--How about telling us your budget ideas;

--Give us a clue as to where this is located...

Get the idea? :-)

Vote #3 for finding an architect that has experience with maintaining architectural character when adding on to a structure.