There is a wide range of options. Depending on the size of the job and the design character/quality you wish, the relative advantages of the choices are as follows:
Find out whether the garden firm 'learned on the job' or whether they have staff who are trained in horticulture or garden construction, or both, and whether the training included a grounding in design, drawing and computer aided design. For simple jobs, contractors will not charge a separate design fee. If, for example, they are building a wall, laying a drive and planting a flower bed, the design cost will be inluded in the quoted price for completing the job.
This is the most common method used for medium-sized garden design projects. Design-build firms will produce design drawings, cost the work and manage the construction. Where necessary specialist craftsmen (eg builders, carpenters or horticulturalists) will be employed by the design-build firm as sub-contractors. Garden owners must be clear about whether there will be separate fees for the design and construction phases of the project. Some garden firms operate like kitchen firms and include the design fee in the overall price. Others charge a separate fee for the design work. Since some clients are not happy with the initial design, or decide not to go ahead with the project for cost or other reasons, the fairest method is to have separate fees for the design and construction phases.
Some garden design firms, and most landscape architecture firms, work on a design-only basis. This is how large projects are handled. The designers produce three types of document to specify the work: design drawings, written specifications and a schedule of quantities. This approach is more expensive for small jobs but cheaper and better for large jobs, for three reasons (1) the designers spend most of their working lives on design, and therefore become expert (2) by putting the work out to tender, a competetive price can be obtained (3) the designers act as intermediaries between the client and the builder during the construction phase, ensuring that everything is done well and a fair price is paid.
Since most garden projects are 'medium sized', we recommend the following as a good way of obtaining expert advice and paying only for the services which the client requires: