However good your computer generated plans are, unless you are one, you will require the services of an architect to draw the official plans in a format suitable for submission to the town planning authorities. These will have building details and standards specified.
Having already drawn up detailed plans of exactly what was required, my architect's job was relatively easy and cheap (around £300). If the architect has to interpret what you require, this may take several passes and cost more. My architect made his own measurements of the house and a survey of the likely route of services such as electricity cables and drainage etc. His plans included details of the depth of the foundations, roof truss design, specification of the stone type and roof tiles. He also handled submission of the plans to the local district council. Happily they passed which marked the end of this phase.
Some architects offer a complete project management service and go on to handle tenders from builders and manage the construction. I didn't feel this was necessary.
If the plans are rejected by the planning authorities, clearly there could be a lot of rethinking and expense. The application of planning rules is somewhat subjective, depends on the 'sensitivity' of the area and varies with time / planning officers. If all else fails join the local Lodge!
The next job was to obtain a builder. This is best done by recommendation and by inspection of completed work if possible. To get value for money, three quotes should be obtained.