In the past people would often have taken the drawings used for the planning application, and used them for building (in truth this has often lead the problems). But with the new regulations clients now have to submit varies detailed drawings, calculations and certificates etc. when lodging the commencement notice to the local authority (LA).
The LA then approves the information before work can actually commence. As part of the information supplied you will have to choose an Assigned Certifier (chartered engineer, register Surveyor or Architect) who will inspect & record/survey the work as it proceeds. At the end of the project a Certificate of compliance must be lodged with a set of as built drawings and details, which again must be approved by the LA BEFORE the property can be occupied.
For your sanity and pocket’s sake the project has to run professionally and smoothly. So my advice is, invest in getting the detailed information together, spend time looking for professional builders and Engineers or Surveyors. Make sure everyone can work together effectively and are singing from the same hymn sheet.