Design of a building is a process. It involves organizational skills based on numerous factors, from functional considerations to material behavior, a clear understanding of building and zoning codes, engineering knowledge, and visual presentation to communicate all this in the form of clear documents - known as construction plans - which are really contract documents with multiple legal ramifications. It sounds as though you have not asked about the difference in cost between hiring an architect and following the method you have described.
I recently met with someone who was actually spending much more for a contractor to provide plans than it would have cost for an architect, and I commented that by working in this manner, she had no control over the redesign of her own home. She met with the "designer" only once or twice, and was expected to be satisfied with the outcome. Since she was not able to fully understand the plans, she was not sure of what she was getting.
I also see a lot of signs that say "design and construction" on them. You should ask questions and check out whether the person who seals your plans is someone you have met with, and that the designer is licensed. Your state can provide you with a list of the people it has licensed, and your local consumer affairs office may be able to help you determine if the person holding himself out as qualified for your project has been the focus of any complaints. Construction is not cheap, and you should not treat the planning process with so little regard.