It's a classic puzzle .
There has not been lot of excavations of hut circles (Thorneycroft ? was one of the earliest) but what little we have had points to them being later than the major monuments , i.e. B.A. rather than Neolithic . There seems to be large swathes of upland monuments and rock art but little evidence of associated settlement and on the other hand great numbers of hut circles and field systems with little in the way of monuments apart from the odd later stone circles e.g. Dartmoor .
The few earlier buildings that we do have evidence for are intriguingly quite substantial e.g. Skara Brae .