If you have a washing area (if you are not using any of the camps toilet facilities) then this should be placed towards the centre of the site as a whole. As it will be used not only for personal washing, but washing the dishes etc. it should be within a short distance of the cooking/dining area.
If you are using natural running water, care should be taken to establish a point from which you will take drinking water. You must then ensure that no-one washes, cleans pots etc. upstream of this point.