Getting planning approval for a balcony is not easy as you are usually impacting on your neighbours’ rear garden amenity space. The Council will normally refuse such proposals on the grounds of overlooking and loss of privacy. Even where there is no possible chance of overlooking, you will fail where councils have decided to ban balconies.
You might improve your chances of success by minimising the possibility of overlooking your neighbours, either by positioning the balcony at the side or by using screens. If the balcony already exists and you have evidence that it has existed for over 4 years, you could apply for a certificate of lawful development for an existing use. This is more likely to be successful than a planning application.