Follow the money

Interior designers take college coursework not too dissimilar from architects.  They have to learn about construction techniques, materials, building codes, occupant safety, mechanical and electrical design concepts, etc.  I don't think they have to take the same level of math or structural analysis that architects must to graduate with a degree. Depending on the jurisdiction, interior designers can make significant alterations to interiors like adding or removing doors and stairs and changing exit paths.   Interior decorators are different.  They are generally not licensed and can only make superficial changes to finishes or colors, and such.

I don't know if engineers bump up against interior designers doing the same work or not, but architects do.  In fact, saying 'interior designer' in a social setting populated by architects is tantamount to throwing a 55-gallon drum of blasting powder onto an open flame.   

"Gorgeous hair is the best revenge."  Ivana Trump

I see.  I wasn't aware there was a difference between "interior designer" and "interior decorator."  

Why wouldn't the architect handle those sorts of interior build-out issues?  Seems a lot like the purvey of an architect to me.

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It depends where you are as in Australia the two professions can have quite a lot of overlap.

My sister is a trained interior designer who is working as an architect and my uncle is a trained architect who did interior design.

In New York the Office of the Professions regulates quite a few.
http://www.op.nysed.gov/prof/

The common tie is sale of expert services to non-expert clients.  

You would choose this person because they are probably cheaper.  Just like using a designer instead of an Architect.

I am working on a job right now where an Interior Designer is the project manager for a remodel on a cliff side mansion and they got caught while building with some Civil drainage issues.  It's not quite as easy talking to an interior designer about drainage and building issues compared to an Architect.

B+W Engineering and Design
Los Angeles Civil and Structural Engineering
http://bwengr.com