Itsallgoingtobefine Can you link me to the relevant bit of planning policy, you have obviously read it. I'm afraid I failed to find that too in my last google. I did find something about planning rules being relaxed to allow farmers a small amount of diversification, but I couldn't find anything about housing on agricultural land
There isn't a link to a specific bit of planning policy that allows building on prime agricultural land, because it isn't something that is considered by current planning policy. Planning policy in Scotland operates on the basis of zoning, mainly concentrated on the edge of existing settlements. In the Central Belt, this often means that prime (Grades 1-3) arable land is zoned for new build housing.
I have read a number of your posts and can't help noticing that you equate a posted internet link with a proven fact. They are not actually the same thing. Things can exist without a link to a random internet site. Things may not be proven by linking to an internet resource. Sometimes, you have to use your own human experience and find out your own proof in the real world and apply it, rather than relying on others to provide endless lists of internet links.
Anyway, to keep you happy, a quick google search produced only a Scottish government circular which highlights the need for it to be taken into account. Given that it was published in 1987 and is of slim advisory value only, its fair to say that it doesn't offer sufficient protection. I've seen hundreds of acres of Grade 1 farmland near my home covered in new builds in the last five years alone. All of this is also vaguely covered by the Town and Country Planning Act 1972, but again, the Scottish Government hasn't seen fit to update it, although it is now within their powers to do so.
I think your approach to understanding and learning is a little unusual, and very much at a tangent at times. You could easily find out this information for yourself.