There are a number of reasons you hire an interior designer, some of which include: a paid point-of-view (you like the designer’s overall body of work); you want a space designed to accommodate your personal tastes and needs beyond what was provided by the architect or developer (or in the case of an older home, what was there…); you don’t have the time required to put all the moving parts of a project together yourself; you’re self-aware enough to know what you don’t know about space planning, the newest materials, why marble or stone is the right/wrong choice for the use intended, how to deal with customs agents and why not to order from certain vendors or foreign countries; or you’re seeking to enlarge your frame of reference (pass West Elm, think out-of-the-box, collect accolades from your circle) and elevate your taste level with a completely coordinated space.
Unless your interior designer is also a licensed contractor or GC him/herself, it is illegal in New York for him/her to direct or engage a contractor that you have hired, even if the designer referred that person. It should be noted that the client, in most cases, is responsible for contracting all trades independent of one another. This could include the architect, the designer, the contractor and possibly specialty trades like decorative painters and plasterers. Hiring a “design-build” firm negates this step, but be sure that they can offer you more choices than “one from column A and two from column B.”
The designer’s role is to make sure that his/her design is being constructed to “generally” conform with his/her wishes. When this is not the case, the designer should immediately contact you and/or the project architect, who in turn, directs the contractor.