Its not so much the max floor area that will determine the size of your site on 1/2 acre. what will determine it is the distance of septic tanks and percolation areas along with the predetermined distance you have to keep back from the road! If you draw a scale sketch of the site and draw up the 20M from the middle of the road, that will give you the front line of your house at its limits. And then off one side of the dwelling you will have to be 15m from septic tank this will give you the area you can build the house in, and after that its up to you really!