There is a grey area between the difference between a landscape architect/designer and garden designer, and you’ll get a different answer from everyone that you ask.
In my view, a landscape architect tends to work with larger projects that can involve multi-disciplinary teams including architects, planners, engineers, services, etc. The projects often involve the planning process, can be long running and may be phased. There tends to be more of a lean towards hard landscaping and ‘easy’ maintenance planting which can be kept by a minimally skilled team. The discipline covers a vast range of project types – road schemes, healthcare and education builds, quarry and mining reclamation projects, retail, residential, industrial and new town developments, the list goes on.
Garden designers tend to work on private gardens, usually of small to modest size. Planting tends to be more interesting or ‘fiddly’ depending how you view it! Projects are less likely to require planning permission.
Many of the world renown garden designers seen at the RHS Chelsea garden show, are in fact landscape architects who specialise in garden design. Some garden designers have designed hospice gardens. So, there is an element of cross over, but in my experience it tends to be landscape architects who cross over to garden design more easily than the other way around.
I am a landscape architect by training and have experience of working as part of multi-disciplinary teams on multi-million-pound hospital and school new builds, and I have designed garden courtyards for small residential developments, and large garden design.