Thats true. But a property cant be conveyed (sold) unless all the structures on it comply with planning laws. So if you have an "unlawful development" on your site and it is there for that long, even though the council cant force you to demolish it, you cannot sell it until you've got retention or voluntarily demolished the unlawfull development, or altered it to comply with planning laws for exempt development (e.g. below certain size). This problem is quite common.