Certifying for the whole house when only something very minor has been done would increase the time and expense no end.
Suppose you were called to put up some fancy light fixture in place of an existing one. What would you check? Condition of the wiring, earth continuity, loop impedance to that fitting, switching in the phase, proper overcurrent protection for that circuit at the board, and that the new fitting won't be placing too much load on the existing lighting circuit maybe.
Now extend that to a major inspection and test of the whole house and you've just turned that job into a whole day's work and more.
