The work involved building a new floor structure across an empty stairwell, removing an internal staircase and applying a waterproofing membrane system to a veranda.
It should have been done by a licensed building practitioner or supervised by one.
Reynolds accepted he was not licensed or supervised but thought the job was not classed as restricted building work, nor required building consent.
The homeowners had no knowledge in the area and had trusted his judgment, Thomas said.
"The bottom line is that tradespeople need to be aware of what they're qualified to carry out, and what consents are needed for the work.''
Breaches of the Building Act 2004 were taken seriously and MBIE would prosecute when required, Thomas said.