is the law to hire an architect ?

Answer 1

In most cases no, if you're talking about a residential addition or building. Depending on local codes/laws in your area, that is.

In most cases the building plans must be seen and/or approved by the architect or their counterparts/coworkers that work for the town or municipality in which you live, to ensure safety and stability of the building. They then issue a permit that allows you to begin demo or building, that must clearly posted at the building site.

But I have never heard of a law that says you MUST use an architect to design or build the home itself.

You do, of course, have to have permits for building it and must pass inspections on site for electrical, plumbing, etc. Check with your local zoning/builder's board or office.