In short
Construction supervision is the body inspecting whether a build is carried out in line with its project and with legislation. Supervision is compulsory, and the body is a legal entity separate from the developer doing the work.
Supervision is not a one-off check. Stages such as foundations, load-bearing structure, roof, plumbing and shell are each inspected separately, with a report drawn up for every stage. Work cannot move to the next stage before the previous one has been signed off.
The independence of the body is the basis of the system: the firm carrying out supervision is separate from the firm building, and carries liability on its own record. Which supervision body stands behind a project is a fact as concrete as the developer's past work.
Why it matters to an investor
Most of a building's quality sits where it cannot be seen once complete: steel, concrete, insulation and plumbing. This is the one thing a buyer cannot measure alone, and supervision records are the only independent source on it.