Throwing light on the sun

How should we decide how much sunlight ought to be enjoyed by existing properties when considering prospective developments? I fear that all too many local authority planning officers are going to stick their arms in the air and yell ‘Please teacher, I know that one. Three hours on 1st March because that's the DoE guideline.’ That is not the answer. In the first place it is not the answer to the question, which was ‘How should we decide?’, but in the second place it is not even the correct answer to the question they will have incorrectly asked themselves, which is ‘How much sunlight should remain to residential buildings after development has taken place on neighbouring sites?’