There are many ways to answer this question. I know, not the first sentence you want to read because you were probably coming here to find a more definitive answer. However, in this industry, definitive is rare. The cost of an interior designer will vary with each project...and oftentimes during the project. It's also important to note that the cost of an interior designer is going to differ from that of an interior decorator. A different blog post on that subject is coming. For now, let's break down some of these costs.
The biggest factor in determining how much an interior designer costs is the scope of work the designer will be asked to do. Most of us have an array of services ranging from very minimal labor to very detailed requests. For instance, my biggest seller is my Room Rejuvenation service and for the most part, that can be done in a flat fee because I am able to accurately determine the amount of time and effort it will take to complete the task. On the other hand, I can't charge my Full Service Design clients a flat fee for one very specific reason.
I have no way of knowing exactly how long it is going to take to complete a Full Service Design from start to finish. There are too many parts involved such as concept planning, client decisions, permit processing, contractor delays and supplier issues, just to name a few. In those instances, fees can be assessed hourly or on a percentage basis, meaning the fee assessed is a percentage of the overall project budget. Those percentages typically range anywhere from 10%-30%. Some clients enjoy this method of billing because they will have a general idea of how much the designer is going to cost. But remember when I said the cost can sometimes change during the project? Well, here it is. If the budget has to increase for whatever reason, the cost of the designer increases as well, You might be asking yourself if that rule applies if the budget decreases too and the short answer is yes, but.... and the but is most designers have a minimum budget they are willing to work for so as long as the project budget doesn't fall below the minimum requirement, they will most likely still do the job.
Now, with hourly rates, most clients understand this but don't necessarily prefer it. That is because, again as I mentioned, it's very hard to determine the amount of hours any one project will take. As such, these projects are often billed weekly, with spreadsheets of the actual amount of hours the designer has spent on your project. These weekly billings will change based on the amount of time the designer has accounted for.
Another method of billing is a combination of fee structures. You'll often find designers who will charge an hourly fee for one portion and a percentage fee for another portion of the project. This is good for a client looking for a designer to simply shop for them. An hourly fee, most likely lower than the regular fee, will be assessed for the amount of time the designer spent sourcing items and then a percentage fee of everything purchased will be assessed as well. Additionally, for some projects a price per square foot fee structure is appropriate. These typically apply to commercial projects and floor plan drafting but can be used as the designer sees fit.
Even with varying types of billing structures, designers are trained to stay within budgets and most times they will choose a structure that best suits the needs of each individual client. As a general rule, clients should budget $50-$500 for consultation fees and an average overall cost of $100-$500/hour for designer labor fees. These will vary based on location and the level of expertise your designer has. While I would like to tell you that design is a one size fits all service, the fact of the matter is it is a custom luxury but well worth it to have an advocate by your side during a very stressful process, access to exclusive markets and a knowledgeable professional to keep costs as low as possible by ensuring things are done right the first time.