My opinion is this, and I reserve the fact that different local planning areas will not always act in the same way.
If you move onto your land on the basis of carrying out building work and comply all along with the requirements of such a procedure, you can then follow one of two routes.
Once the building work phase has ended, you can apply for an agricultural dwelling but to achieve this you will have to show it is necessary to live there 24/7. You will also have to demonstrate it is sustainable and this will require a proven business plan or previous accounts showing such.
If you have been running a financially viable business whilst doing the building works (as well as fulfilling the functional need to be there 24/7, you could likely get a permanent status. However, if you cannot show financial sustainability, the likelihood is that your planning department will grant a temporary period for you to show such feasibility, i.e. a three year period.
Due to the introduction of the NPPF, it is my feeling, rightly or wrongly, that the financial requirement could be argued to a lesser degree prior NPPF, but that is a matter of how the application is presented and your planning department. As we have not yet entered the time of pure NPPF days, there is no real way of determining what will be required yet.
Thus we are in a grey period where we don't really know all the answers and neither do the planners. What is clear in the NPPF is an essential need to live on the farm (or the functional test, as per the old days) and that sustainability, although not mentioned in direct relation to rural dwellings, is still a general requirement as shown within the wider context of the NPPF.
To get an 'instant' agricultural dwelling to my knowledge, has only been achieved on large scale poultry practices, where tens of thousands of birds are kept and running to a business model with a contract such as those provided for by the major packers,,,, and that sort of practice gets no support from myself.