I'm no Adam Smith scholar but know he talked about the hidden hand of market forces etc and how that affected the price of bread etc but not sure he talked much about planning policy. Perhaps you can enlighten us.
I haven't visited Smith for 40+ years, nor have ever been accused of being a scholar of any type. However, the concept of property, real property and the underlying bundle of rights in particular, have been of great interest to me for longer than that.
AS didn't theorize as much as observed that wealth is created through the use of private property to produce income greater than the cost of production and the owner's personal consumption. Property rights incentivized and enabled their holders to more efficiently allocate resources through the operation and discipline of markets. I can give any number of examples where stripping the rights of owners to enjoy their property resulted in the rapid decline of property, the economy, and society at large.https://www.youtube.com/watch?v=jSuETYEgY68
I don't know what AS thought about planning policy specifically, though I suspect that he was not a fan of central planning in conflict with millions of people making individual choices as to the type, price, and quantities of bread (or most any product or service for that matter) they chose to purchase. He would not, for example, be surprised that the care of the Coul Links SSSI has been neglected being that the private owner is essentially prohibited from using his property for gain.
A superior approach, IMO, would be for the government, in the interest of the community and the country, acquire the designated land from the owner AND assume the stewardship the SSSI deserves. A person with your skills and talent can assist all stakeholders in determining the fair market value of the land being taken (it is done every day in eminent domain cases; in the U.S., if the owner and government entity can't come up with a mutually agreed value, the owner can file suit in district court, I think, and sometimes prevail at a considerable higher price than paid in the taking).
As to the designation itself, my bet is that it is much more of a political than scientific process. And, as we know, that can change in a dime.