Planning applications are required to deal with defined plot areas, you must submit the actual site location and area plan. The difference between the 0.5 and 0.6 acres may affect the area available for percolation of the foul treatment plant, the plot ratio for the area etc. I would strongly advise you to discuss your planning application with the local planning authority before you go any further. They are required to meet you in a 'pre-planning meeting' and you can raise the site specific issues on the basis that you are a potential legal purchaser of the site with a view to developing a one off house on it.
by mr june