In order to call yourself an "Architect" or advertise as such, you must be licensed in the state in which you practice. To use the term "Architect" in any advertising form such as business cards, company logo, etc. is illegal if your are not licensed. Architects at minimum attended an accredited university (the program is usually 5 years for a bachelors degree), worked under a licensed Architect as an intern for a specified amount of time, and then completed an state licensing exam.
A "building designer", "residential designer", "home designer", in simple terms are unlicensed Architects. The law in most states do not require a licensed architect to design residential buildings. Building designers may have attended an architectural program and not tested for a license. Some building designers are field trained through employment under a building designer or architect. Others may have attended university programs that are not accredited such as the Architectural Engineering program. The Architectural Engineering program covers many of the accredited architectural program courses which includes architectural history, estimating, strength of materials, architectural design and working drawings (residential and commercial), construction practices, structural design, solar heating and cooling, mechanical systems, electrical systems, codes & specifications, and surveying. Building designers who studied under this program are well trained in designing buildings and usually focus solely on residential services that include stock house plans, custom home designs, home additions, and home remodeling. However, the program is only four (4) years in length and is not accredited. Building designers are able to design light commercial buildings also. Each state, city, and local county has its on laws that address the limit on size and other issues. Some states restrict all commercial work to licensed architects.
There are two organizations in which building designers can achieve a level of public professionalism. They are AIBD (the American Institute of Building Design) and NCBDC (the National Council of Building Design Certification). AIBD is to building designers what AIA is to Architects, an organization of professionals bound together in a common interest and expertise. In fact, AIBD include architects within its membership that specialize in the design of house plans exclusively. Both groups require a certain amount of CEU's (continuing education units) each year to show that you are actively learning and staying in tune with the latest in design technology. NCBDC is a seperate organization that oversee the national certification of building designers. House plan designers looking to become nationally certified will have to pass a two (2) day certification exam involving eleven (11) sections. If successful, they become CPBD or a "Certified Professional Building Designer". This is the highest accolade that a residential building designer can abtain. As of now, it is not required by law for building designers to be certified but, this may very well change in the near future as several state are moving towards regulations on the residential design industry.
Building designers or home designers, as referred to by some, do great work. In some cases, building designers are even better than some architects that specialize in other types building designs. This is not to say that architects are not good home plan designers, but you may consider choosing your design professional based on his/her speciality, experience, talent, and not just licensure. You will likely encounter various home plan design fees with either of these design professionals, so you must also consider your design services budget too.