We only move on to the detailed design stage when client and architect are confident that the project is meeting all the requirements of the brief and is estimated to be within the client's stated budget. Generally this stage would not be commenced until all planning consents had been obtained.
When necessary other consultants would be brought in to the design team. Even on a small project it would probably be necessary to seek advice from a structural engineer.
At this stage in the design the drawings being produced would be used for Building Regulation application and for obtaing prices from contractors and ultimately for construction