I suggest that you apply for a pre-application if your proposed project has a contentious or high risk element. It can be a low-cost exercise if you choose to provide the planning officer with minimal detail. However, by providing alternative options with a higher level of detail in the drawings, you may extract a greater level of feedback from the council.
The planning officer will issue guidance on the likelihood of you gaining approval. This guidance will be examined at the eventual application stage and may have a bearing on whether planning permission is granted.