Engineering and architecture are governed by private member bills which delegate the administration of these trades to their individual professional associates to establish minimum standards for membership/practice, code of ethics, and enforcement.This is not an issue for the building code. In the United States such issues are defined by the state legislatures.
A generous interpretation of building code provisions referring to design professionals is that they are attempting to restate state laws that govern the practice of engineering or architecture.
When the building is other than a single-family residence to be occupied by the designer, I suspect that there may be other constraints.
Even if an individual is legally able to design his own home the reality is that the process will typically go smoother if the owner hires a design professional to assist.
Laws limiting who can perform acts, such as licensing laws, are governed by the antitrust laws. What makes these state laws legal is the state action doctrine which requires that the state legislature, not local jurisdictions, must clearly articulate a desire to restrain trade in order not to have an antitrust violation.
I cannot speak to what is the law in Canada.
This would not apply here as Ontario uses the Ontario Building Code, which is based on the National Building Code of Canada.2015 IRC section 106 speaks to construction documents. In Massachusetts we allow anyone to design 1 & 2 Family homes, under the prescriptive allowance of the code. If the design or structural element fall out side of the prescriptive tables then a RDP needs to deal with those elements.
We have separate licensing provides of those who construct for hire, however we allow the home owner who intends to live in the home to act as the general contractor, assuming responsibility for compliance with the code.
As Mark K sated it would be up to the jurisdiction and laws to amend the code requirements and set licensing of trades persons.
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