All garden design, whether by a professional or not, is an exercise in arranging space. It is the way in which the available site is divided and managed that dictates whether it will succeed as a design. Some people are happy with the haphazard result of piecemeal changes to their gardens - and this might even be at the scale of individual plants, added as and when they are acquired - while others need to control their space more thoroughly and work to a plan. I would argue that even if you are the type to let the garden evolve around you in spatial terms, a ground plan is still useful.
The space we have to play with is governed by three planes. The ground, which supports structures and plants, is the key to a strong design, the vertical planes surrounding the space are the place to play with the apparent distances of the boundaries and the overhead plane, often forgotten, is where the gardener can manage the quality of light and enclose the space more fully.
The ground has to be divided up - paths, terraces, planting, lawns and water all have their place here, and to manage the space successfully they must be in balanced proportion but should also relate to each other in a coherent way. There is no point installing a lovely place to sit if you can only get to it by wading the pond or trampling the flowers. Pathways should be wide enough to cope with the number of people likely to use it, as well as the mood you wish to evoke - will the path be wide enough for two to walk abreast or will it be designed for single-file use, increasing the sense of adventure, anticipation and adventure? Terraces need to be sited within the space to make the best use of the amenity most in demand - in a British garden this would invariably be to maximise exposure to the summer sun, in a Spanish garden to make best use of the shaded areas. Are lawns and plantings arranged where they will thrive, or do special types of plants need to be used in order to clothe the ground? Have the routes of flow been considered, and are the recreational spaces big enough?
The walls, fences and hedged edges to the plot can seem nearer or further, depending on which is used and how they are planted up. Used within the space these verticals can screen off areas of the space for practical and/or aesthetic reasons, and can make a powerful contribution to the mood of the garden - it's always good to have an area that is not immediately visible or apparent to the visitor: mystery and surprise are everything.
The overhead plane can be controlled or indicated by trees or overhead structures - arbours and pergolas, sunshades and beams. Two main purposes are served - there is the possibility of introducing shaded areas if these are needed, and there is a subtly implied enclosure. The endless space of the sky is divided just above head level and, in wide open spaces, comfort is derived from an implied human-scale space.
If all of these planes are sensitively managed, and the design fits the requirements of the users, then the only thing to worry about is the planting, and there are whole encyclopedias about that!
The photograph shows the grounds of the chateau at Amboise, on the Loire.