The other responses were excellent and I won't repeat the same information. I wanted to ask if the 2-story addition will fill your need? Are the bedrooms upstairs? Usually, in-law quarters might include a bedroom on the 1st floor. If if does meet your needs, I'd suggest designing the space so that you could re-capture it back into your existing home.....5th bedroom, bonus rooms, etc. with defined uses. At the point of your selling the home, you could choose the best marketing strategy based on existing markets. For example, is the market for in-law suites expanding (when you need to sell), or are you better off creating one large home with bonus rooms?
I've also seen very nice garage conversions. The garage usually attaches to the mud room/laundry and kitchen. The garage uses I've seen are one or two room spaces for living quarters and bedroom/bath. This would give your parents privacy, yet quick access to other needs You, of course, would have to park in the driveway or on the street which may or may not work out in your community.
RE/MAX at Jennersville