This is one of the more common questions I get asked. Well, it is probably the most common question people want to ask, but never quite do, as they don't necessarily know how to phrase the question. Or, they're afraid of seeming like they are asking a stupid question. Have no worry, there are no stupid questions about this issue, as it is confusing.
Here at Board & Vellum, we are what's called an "integrated practice." That means that the separate-but-linked fields of architecture and interior design are both practiced here. The design process is often the same for both paths. We have architects who only do architecture. We have interior designers that primarily select finishes and furniture. We also have people who do both and walk that line. It also means that if we are working with either an interior designer or an architect outside of our firm, we like to work together with all of the design consultants from as early as possible. This ensures a project that looks and feels cohesive. Sounds great, right? But again, what the heck is the difference?
Let me try to clarify the muddy gray waters of this issue. I can't promise clarity, but I can hope to clear up some issues.
In the most basic form, when you hire an architect, we are looking at not only the forms of a space, but also the big picture pieces, such as life safety issues, engineering, exterior materials, and the big code questions. For a house, that means what the exterior looks like, how the house performs from an energy point of view, handling building permits, and often how the interior casework and cabinetry lays out.
When you hire an interior designer, he or she is typically going to handle the parts of a project that deal with how the interiors look. This can include selecting interior finishes (tile, hardwoods, carpets, wallpaper), as well as selecting furniture and soft goods (pillows, accessories, etc.). It ideally includes working with the architect on the layout out of the spaces, and how the cabinets, furniture, and other considerations work within the layout. This ensures that windows are properly placed, rooms are sized appropriately, and that the whole space feels harmonious.
In the middle gray area, are the often ambiguous services that many architects and interior designers both perform. This includes selection of interior finishes, such as tile and woodwork, drafting of interior elevations and details, and space planning for furniture placement.
Architecture vs. Interior Design – Venn Diagram of Architectural and Interior Design Services
I'll add that the term "interior decorator" is a much smaller subset of "interior designer," and is more about the selection of soft goods, furnishings, and window treatments (and often equally valuable as a consultant).
If you step back and look at the room you're in right now, you can imagine that there are a lot of decisions to make when planning out a space. Everything from where the windows are, to selecting that pillow on the couch, are decisions that need to be made, especially in single-family residences.
Our process is fairly à la carte, as so many clients have a spectrum of interest in the process. Some clients are hiring us to make every single decision, while others really want to be part or sole owner of the selection of interior finishes and furnishings. Often we'll have a client who has a great past relationship with an interior designer, and will bring that person to the table (hopefully early on), and we always welcome that working relationship. In the end, we feel it is our client's home and helping them find a path to get a great finished product is our job no matter what level of interest or variety of design consultants that they want to bring to a project.
As you think about your project, you should consider what services you think you'll need or want, and then assemble a team that has the ability to pull it all together. Carefully analyze all of the parts of your home that you'll want handled beyond the basic design, and see what your consultants can do and how that is billed. (Often architecture and interior design services bill a bit differently). If you think ahead it will be a better project that looks cohesive and wonderful.
Source: https://www.boardandvellum.com/blog/architecture-and-interior-design/