Research well your designers prior to hiring. You can use draftsmen or architects to create your plans. Usually draftsmen will be more economical. Ask others who may have recently built or added on to their homes who they used for their house plans and what they thought of their work.
Some designers figure an amount of allowable revisions. Since we are adding on approximately 700 square feet, we did not need to go back and forth too much with the designer. It was a different story with our home. When we worked on the plans for our house, we had many, many revisions. Thankfully our designer was tolerant with the back and forth, but many may not be. Be sure to ask about this when you’re interviewing designers.
I recommend sketching things out on paper first and deciding on details before hiring your designer. Another thing you can do upfront is send your designer a few, (not an entire album), of online plans or magazine pictures depicting what you are interested in building.