If you are purchasing a field of say 10 acres at agricultural prices (with regard to location) say 10 - 14k per acre, then you would not be taking much of a risk buy completing the deal prior to applying for planning as if it was refused you would still have a field which would be worth as much as you paid for it. It is more usual when buying a small plot of land specifically to apply for planning, that you buy subject to planning. The fact that it is for all intents and purposes a site (because the deal will not be concluded unless planning issues) will be reflected in the price ie you might contract to purchase 1 acre in a country area for say 70k subject to planning. Without planning 1 acre would only have a agricultural value of about 10 - 14k.
The bank may give 80% but 50% is more usual, especially if supporting security isnt being offered. The reason for this is that you could let the planning lapse and then it is just a field - Bank security gone. If I was giving someone a loan to buy a site I would want to know that they could afford a mortgage to complete the construction of the house or that they had sufficient equity in their present house to finance the build.
The bank may give 80% but 50% is more usual, especially if supporting security isnt being offered. The reason for this is that you could let the planning lapse and then it is just a field - Bank security gone. If I was giving someone a loan to buy a site I would want to know that they could afford a mortgage to complete the construction of the house or that they had sufficient equity in their present house to finance the build.