General

On the face of it the UK system regarding the construction of houses, extensions etc seems pretty much ordered and regulated.

WHAT HAPPENS IF THOSE IN AUTHORITY MAKE A MISTAKE.

mistake meaning

1:  a wrong judgment : misunderstanding.
2:  a wrong action or statement proceeding from faulty judgment, inadequate
     knowledge, or inattention.

If you want to build a house or add an extension on an existing house you will have to involve others to inspect your work.

Building Control is a legal requirement that must be obtained before carrying out work that involves the erection, extension or alteration of a building.

You can apply to any Local Authority Building Control department or Approved Inspector for building regulations approval. Those carrying out the inspections have to be qualified and they will make statutory and routine inspections during the construction stage of the building work to ensure compliance with the building regulations and other allied legislation and guidance.

Depending on the size and complexity of the works, you may need to make a Building Notice application or a Full Plans application. In both cases, you need to infom the relevant authority at least two days before work starts.