When you build a room in a house, you need a detailed construction design so you will get what you want and not waste time and money.
Outdoor rooms are the same.
A design is economical. It will save you time, energy and money. You can take as long as you need to fulfill it, and you will always know what your finished garden will look like. A design will locate the right plants in the right place. They will flourish and not sprawl over their neighbours, or leave gaps that Mother Nature will fill with ugly weeds.