The simple answer is maybe you don't, but then again maybe you do. Consideration of complexity and magnitude of specialization should help you make that decision.
 

DESIGN

What is DESIGN? In relation to a new home or a remodel / addition project to your residence, Design is the process of making sure that your ideas are implemented into the building and harmoniously interrelated to create an overall appearance and aesthetic feeling.  This process begins as you first start thinking about a construct project, envisioning its appearance and beginning to define the building's constraints and possibilities.  You have begun the design process. It is finished when the last bit of work is accomplished on the project and you use the building for its intended purpose, hopefully in the way and manner that you envisioned in the beginning.
 

THE ARCHITECT

The design professional channels that effort into a structured system and provides you with a trained eye and to aid in assuring that your needs (the program) are met in the design. The design process as Me mentioned  begins with the seed of the idea and concludes when the job is finished, but the majority of the effort must occur prior to the onset of construction. The architect has the ability to see the problems might occur with a given design concept and therefore reduce the number of changes that would occur construction. He also is able to coordinate the efforts of other professionals (energy compliance consultants structural and civil engineers, soils and surveying consultants) and perhaps more important, he can deal with, understand and interface with the government regulatory agencies in an informed and professional manner.

Another key element in the architect's sphere of influence is his ability to work with a general contractor, typically someone he has worked with in the past and shall do so again. He is able to interpret the contractor's concerns and present them to you as an impartial judge of the situation. Due to his knowledge and ability to refer future work, the architect is able to prevent a construction problem from becoming an opportunity to inflate a Contractor's profit on a project.
 

OKAY SO WHAT IS THE ANSWER?

So to answer the original question - maybe you don't need an architect, if you can foresee the consequences design solution, see options for other designs, feel comfortable dealing with the regulatory agencies, other general contractor and understanding the building and construction terminology and implication a given course of action.
 

OKAY!  OKAY! - SO WHAT DO I DO NEXT?

If you decide to hire an architect because of the size of the project, your desire to have it done the best way0 or your need for professional help - the next question is - "How do you hire this person?" The answer:  Talk to several people, talk to their references, understand how the design process works in their offices and what the interrelationships will be between you and the architect. Ask who will do what and when and at what rate.  Understand compensation and if you are protected from cost overruns and to what extent. Evaluate his work and ongoing workload in relation to the desires and needs of your project: Does he specialize in type of project you are planning? Does he understand your needs and is he flexible with the design process outcome to meet your ultimate desires? Always remember you will live in the home and he will not - you have to live with the outcome. Be part of the process and make sure the architect wants the same end results do.