First is the question of whether the plot lies in a village or in a town area where your council’s local plan indicates that new house building can take place. If so, then the new houses must have adequate access and parking, be compatible with neighbours in terms of design and the character of the area and not give rise to a loss of privacy. The new houses and the original must have adequate amenity space (garden). You don’t need full plans to make an outline application, but you will need to provide a layout plan showing the footprints of the new houses and how they will be arranged on the plot.