So what are the building regulations then?
If planning is about what you want to do, the building regulations are about how you are going to do it. You are required to demonstrate, using one of two methods, how you are going to construct a safe and habitable building. It is all about concrete, lintels, steel beams, insulations, timber sizes, and so on.
One of the methods is called a Building Notice application, really only suitable for internal alterations and smaller projects, (porches for example).
For all other projects a Full Plans application is more usual. It involves a specification that is checked and approved by a building control authority before construction begins, the contractor builds to the approved specification. Specification checking and site inspections as construction proceeds can be by your local authority or by a private sector company.