Nothing to do with values having fallen. Quite simply the block which was bought at land value or thereabouts is worth $100 000 less because it can't be subdivided. That applies today and also applied when I purchased it. It simply was not worth the asking price because of this and a BA who was doing their job properly should have picked up on this - that was their brief.
Go and fish for the PI insurance.
if its documented that this is what was required, and you have good documentation that clealry says it is subdivisable according to your brief, and you acted in reliance of that information, and you have suffered loss as a result of that information being false, then it would be worth having a chat with the BA for a start, and if that doesnt tale u anywhere, then a good solicitor to see if its worth taking on
ta
rolf