Unlike a house, garden design changes from one week to the next. Plants grow, bloom and die back, sunlight moves across the area, seasons come and go. For many, this element of change and surprise is the main appeal of gardening. Certan plants may be quite spectacular in one season yet unobtrusive in others. Colour schemes, views and density can change from season to season too.
Planning is essential to an attractive, manageable garden.
If possible, plan on paper first to avoid wasted time and money. Minor adjustments can be made later, of course, but major features like walls or walks tend to be permanent, and many plants suffer from transplanting and take years to recover to their original glory. In addition, planning on paper first makes it easier to plan the garden as a whole rather than tackling it piece-meal. This helps to create a sense of continuity and harmony between one area of the garden and an other. Take your time.