Go and talk to planning and they should be able to give you some pre-application advice. Whether they will stick to that come application time is another thing!
Get the planning/survey drawings drawn up and off the back of thosethe building regulation/construction drawings can be done. In terms of structural engineer they would need to confirm the foundations, floor joists and possible roof trusses. Drainage can be done by architect/technician/technologist but would be confirmed by the BCO when on site.