What type of garden do you have? Are you wanting landscaping around your house or do you have a larger project? What type of landscaping did you have in mind (formal, cottage garden, etc)? These answers would help ensure you are given more useful advise.

If you're needing help with hardscapes, drainage issues, retaining walls, excavations and so forth, you do need a LA. But if what you need is good landscape design and knowledgeable plant choices, you may not. Landscape architecture is an engineering degree rather than a horticultural degree, and a LA I know told me there were just two required hort courses, which he described as rather basic, to get the degree. As a consequence many LAs have a rudimentary knowledge of plant material. In my experience they often spec plants into a plan from pictures in a book.

Which isn't to say that there aren't LAs that do know plants. Some really know their stuff, and you can find this out during the interview process with whomever you consider hiring.

A good landscape designer may be all you need, though, and they usually come much cheaper than a LA.

Marty