I love old houses, and gypsyheart is right .. all areas should be hunted, over and over .. and from many differnt direction, so that you can get good overlap. Go slow and listen for the deep signals.
Grass will do better if you hunt after a rain, and some people think you can pick targets up better and deeper when the ground is still wet.
Some of the hot spots for old houses are, clothes lines where clothes wear hung to dry. The paths to the out houses, remember outhouses would be moved from time to time. Also digging the old outhouse pits are good to many things including old bottles were disposed of .. or lost in these pits.
Also before we had A/C, people would sometimes sit out under shade trees on hot afternoons. Just remember trees from 1700's most likely are not there now. Another place might be in and around outbuildings, people might have layed on the ground to work on wagons, cars or trucks.
Places where the kids might have played .. sand boxes, tree forts and such are always good.
Let us know what you find .. good luck
Old Tom