the planners will be looking at the size, materials used, design and landscape setting of the proposal. The size should be appropriate to the current house - keep the proposal in scale with the current house, and appropriate to the surrounding houses on the street. The materials should match those of the current house and those of the surrounding buildings. The design should be aesthetically pleasing and not out of character with the existing buildings. Landscaping - make sure you check for any trees with preservation orders and don't harm them if there are any. If you are in a conservation area or green belt area there will be stricter design guidelines. Also, keep good relations with your neighbours as they can be your downfall when it comes to objections and remember neighbours that see the house from another street or from behind e.g. a parralell road of terraces.
best thing to do is to ring the planning department and ask to arrange to see the planner that deals with your area, then you can chat to them about what is most likely to be approved and what your chances are. draw up some professional plans - include a site plan showing the boundaries of your property and where the proposal will be, and existing and proposed front/side/back elevations. The planner can take a look at your plans and advise you before you submit an application. Or, you can submit for outline planning permission which asks 'in principle, would the idea of this proposal be acceptable'? and then you later convert to full planning permission by submitting more detailed proposals.