Ally,
I am not so sure. Though there are some cases where this is the case taking that behind means seaward side then there would be no problem in building on existing dunes as new ones would be forming all the time to replace them. However, dunes are an unstable and transient landforms. They are created over a number of years or decades and are more than often eroded away . It is only in modern times that we have the idea that the coastline is fairly fixed in shape and form but this is not the case. At the moment, the long dune system stretching to the south of Dornoch is increasing in volume with a few points of erosion but further up the coast at Golspie, the coast is being eroded at a fairly quick rate. This has bee the pattern for the last few decades but that is not to say the process will not completely turn around in the next 20 years or so. But then again maybe not.
Despite lots of study we know the 'what is happening' with coastal changes but we do not really understand the mechanisms so can not predict with any certainty what will happen in the future.
Jon