Most people will likely go their whole lives without ever hiring an architect. Others will hire an architect because of a perceived need. Yet others will hire an architect because of a real need. So, when do you have to hire an architect?
Well the short answer, at least in OHio, is never. There is, of course more to it than that, but from a building code point of view, there is never a situation in which you would be required to hire an architect. Let me explain...
Most residential projects are not required to have an architect involved as they do not require sealed drawings for permitting and are regulated by their own code. Yes, you still need to get the plans approved. But anyone can produce the requisite drawings for a permit so long as the meet certain standards and contain certain information. Your builder, a plan service, a residential designer, or even yourself are all acceptable generators of permitting drawings. As such, the remainder of this post will deal strictly with projects that would fall under the jurisdiction of the Ohio Building Code, not the Ohio Residential Code.
Commercial projects do not strictly require an architect either. I receive a fair amount of calls because someone is looking to do a project or have already started construction, only to be told that they cannot proceed without hiring an architect. That is not exactly true. If you are looking to do a commercial project, you are required to engage the services of a design professional. Which could be an architect. Or it could be an engineer. So long as the professional is registered / licensed to seal the drawings.
That being said, there are certain instances where a building permit would not be required even for commercial project. In those cases, an architect would not be required. However, nearly all commercial projects fall under at least one of the conditions for requiring plan approval. If you are planning on constructing, altering, moving, enlarging, replacing, repairing, or maintaining a building, you are likely to need an architect (or other licensed design professional). Even if you just want to move into an existing building with little to no alterations, you may need an architect. The Ohio Building Code Section 105.1 requires that:
Any owner or authorized agent who intends to construct, enlarge, alter, repair, move, or change the occupancy of a building or structure, or portion thereof, or to erect, install, enlarge, alter, repair, remove, convert or replace any electrical, gas, mechanical, plumbing system, other building service equipment, or piping system the installation of which is regulated by this code, or to cause any such work to be done, shall first make application to the building official and obtain the required approval.
Furthermore, Section 106.2.1 requires that "Construction documents shall bear the seal of a registered design professional..." There are a few exceptions, but they are mostly not applicable.
Basically, if you are doing anything more than cosmetic work, a design professional will be required. Constructing a new building? Hire a design professional. Changing the use of a building? Hire a design professional. Adding on to your building? Hire a design professional. Cutting holes into or moving walls? Hire a design professional. Changing egress routes? Hire a design professional. Adding light fixtures or receptacles? Hire a design professional.
The Richland County Department of Building Regulations has put together some really nice resources to help you determine when you would need to hire a design professional that can be found here.
DISCLAIMER: The above represents my understanding of the law in OHio. However, the law is a robust and complicated thing. And i am not a lawyer. As such none of the above comments are intended to be construed as legal advice. Check your state laws for the rule pertaining to your location.