There are many articles written on this subject and I don't want to repeat all the words of wisdom you can find on many sites!
Our office does both types of work.
Many projects are small: decks,pergolas, small extensions and even residences. The extent of drawings required re the legal requirements and structural documentation is no less important for a pergola as a whole house. The importance of the right layout, orientation, weatherproofing, structural integrity, materials etc is just as great which ever project. So often it is experience and expertise that makes the difference.
Whenever the owner has a builder already or is confident in working through the project with a builder, it means we only need to supply the basic drawings to show the details of the construction and to obtain a building permit. Many e owners are happy to work through choosing all of the appliances, services, finishes, tiles, cabinets, colours, lighting etc and work through the project with the builder. Obviously, a good reliable builder and trades with experience will make the process easier.
Our office does many of these projects at a competitive rate for homeowners and developers. I think it is important to offer a service no matter how big or small. Often this work is good to help train up young designers.
However, when you are spending so much money on a residence I think it is worth getting the best expertise and taking the time to get the best value from the building process.
Taking time at the planning and design stage will help to achieve the best results. Many times it will be on the fourth or fifth design variation that the real quality design begins to emerge. if it is a budget design process, there is no money for that much exploration of possibilities. If an owner is looking for something unique, that also requires detail drawings. We have an interior designer on all our top projects, and it is amazing the level of quality that can be achieved with good selection of fittings and finishes.
n Victoria, Building Designers need to be registered and many are very capable in a number of fields. Architects must be registered to use the title and go through at least 5 years of training and then are required to complete competencies and experience in the many fields of expertise in the process of design, documentation and construction of a project.
For further detail on the differences see the article written in Build entitled "What's the difference between an architect and a building designer?- http://www.build.com.au/whats-difference-between-architect-and-building-designer
Ultimately it will be up to you and what you need, and on who will suit you for your project?
Pictured above is the steel frame installed ready for the timber treads. Detail like this does not just happen it is a combination of Architect, Engineer, Builder, Steel Fabricator all working together to achieve something spectacular for the homeowner.