With effect from 1 April 2018,
It is an offence for anyone to engage a plumber who is not licensed by PUB to carry out regulated water service and sanitary plumbing works.
PUB (Singapore's National Water Agency)

Types of Plumbing Works

A plumbing system refers to the pipes and fixtures installed in premises/buildings.

There are two types of plumbing systems in all facilities: one caters to the supply of drinking water, while the other takes care of the disposal of used water.

Types of plumbing works are:

• Water service plumbing works refer to the laying, installation, alteration, repair or replacement of water pipes, tanks, taps and fittings in the drinking water supply system, for the conveyance of water supplied by PUB. 

• Sanitary plumbing works deal with sanitary discharge pipes and sanitary appliances (water closet, urinal, sink etc.), such as installing, changing or relocating toilet bowls and sanitary piping system, for carrying used water from the premise/building to the used water system.

All regulated water service and sanitary plumbing works must be carried out by Licensed Plumbers (LPs), except for simple plumbing works that can be carried out by handymen.