Everybody's taste varies and different styles will suit different types of property. Nevertheless, a well-designed building or extension is likely to be much more attractive to you and to your neighbours. It is also likely to add more value to your house when you sell it. It is therefore worth thinking carefully about how your property will look after the work is finished.
Extensions often look better if they use the same materials and are in a similar style to the buildings which are there already. It is impossible to give a single definition of good design in this context: there may be many ways of producing a good result. In some areas, the council's planning department issues design guides or other advisory leaflets which may help you. You may wish to consider using a suitably, qualified, skilled and experience designer.