An enclosed patio differs in that it is a fixed structure capable of dealing with the elements, unlike a retractable awning which can be rolled up in winter or severe weather conditions. A good example of this sort of structure is a carport or covered walkway. So how to enclose a covered patio is an area worth investigating if you need more space, be it for the car, or a games area where the children can play all year round.
If you already have a porch, walkway or carport, by simply screening it you will have an additional room, great for those days when the BBQ has to be cooked indoors because of the increment weather, however not terribly suitable in the cold of winter.
If you require an all season room it is possible with some extra work and added expense. You will need to have windows installed. This is a good way to add living space to your home add will increase the useable square footage.
You need to examine what it is you need as the cost of enclosing a patio can vary enormously depending on what you require, so decide if you are going for a screened porch or a weather tight room with windows. Also, think of your garden decor when deciding these things.
You will then need to think about the roof, is the current roof going to be suitable (if you have one). Would you like a hipped roof or a simple angled roof that butts up to the side of the house? How will the roof be supported is a primary question to ask yourself. Also in an enclosed room you will have to consider insulation, both walls and flooring, importantly do not forget the flooring. The floor is crucial when planning an all season room.