For a loft conversion, you would generally appoint a designer to organise or do the following:

Complete a measured Survey of the existing property
Commission a design to deliver your brief
Prepare a set of Planning Drawings (plans and elevations)
Apply for planning permission or prepare an application for a Lawful Development Certificate if the project can be built under Permitted Development rules
Once the application is approved, prepare the Building Control drawings, liaising with the Structural Engineer
Obtain contractors quotes