In other words, what you have, what you need and what you like. Additional foctors include the size of the area, style of the home, time available to maintain your design, and other available resources, like garden supplies and nurseries. 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.