Building Act changes now in force mean building and design work critical to a building's integrity can only be done by a licensed building practitioner.
The Department of Building and Housing says the introduction of restricted building work involves making houses structurally sound and weathertight.
The department's registrar of licensed building practitioners, Mark Scully, says designers and builders will have to have to demonstrate their competence in order to do restricted building work.
Master Builders Federation chief executive Warwick Quinn says allowing only license holders to do such work will protect the public from bad workmanship.
A licensed builder from Wellington, Rob Clarkson, says the licensing regime should root out the cowboys.
Graham Hollobon, who owned a leaky home that was so badly built in 1990 that it later had to be demolished says the new rules are more than 20 years too late.
The changes came into effect on Thursday.