As an architect I can't imagine building a $1M plus home that hasn't been designed for the way I live. A good architect will walk you through the process, step by step, steering you clear of problems, design your dream home to fit your lifestyle, save you thousands if not hundreds of thousands of dollars on construction and yes, we've had to do a little marriage counselling on occasion
If you buy a set of plans, it's not a custom home.
A custom home designed by an architect will take into account the positives and negatives of your lot; views, orientation, solar potential, value vs. quality, potential noise just to name a few things.
Here's the most important thing people overlook when they are building a home. Square footage. How big do you really need to build. If you can shave 10% of the floor area through good design (which is easy to do), you will have saved every penny and more that you've paid a professional. I've seen so many homes get built from standard plans that are 10, 20 sometimes 50% bigger than they should be. The point is, design to fit your lifestyle.
Every custom home I've designed has been appraised after it's complete at a value that is higher than the owners expected and higher than the cost of construction + my fee's.
Here's one simple thought that I tell every residential client I work with. Tell yourself that this is not the last custom home you will build. It makes the decision making process incredibly easier because you won't agonize over every detail thinking that it has to be perfect. First instincts are almost always correct.