Keep your focus
Avoid going to the grocery store on an empty stomach.
Create a shopping list and try to stick to it.
Often I ask my clients to list what they want, and during the discussion, the project can grow substantially. While this is a valuable process to discover what may be more desirable to accomplish, we typically scale back before pre-design phase, sometimes with a modified goal.
I suggest that before an initial design meeting, you create two lists. The first list should be bare bones and contain just what is essential to the project. This should be very tight, strictly adding only what is needed or strongly desired.
With the short list complete, now you can dream a little and add to this with items that may make your life a little more fun, and that includes final touches or a desired style. You will be at this location often, and it should be a joy and draw you in, especially if it is a place of work or relaxation and not completely utilitarian. Many of the decisions you may make can be emotionally driven, and this is nothing to fear, as what you are creating should make you happy and life easier.
Once you and a designer get together, more may be revealed from the experience that a professional brings to the table. Your designer should be an asset to the project and be informed on regulations, methods and materials that will make for a better build. Experienced designers don’t come cheap, but when they know what they are doing, the expense will pay for itself in the construction process, and then some.