It sounds like you constructed a level addition that did not match up entirely with the existing house. 1/2" is not a lot of height to match up, but it is perceptible under carpets and wood floors. You need to fill the low spot and feather it out over as long a distance as possible. I'm not sure, but floor filler may be appropriate here. You would want to match the height on the high spot of the floor and screed the level back 3 or 4 feet.
I haven't dealt with this, so lets see what the others say.