What is the best way to extend a room at the back of the house?

I'm looking to add some additional space to my flat by extending the room at the back. It is single storey and already has double doors out to the garden, I literally want to add another square of space to it. What is likely to be the cheapest way to do this - brick, glass or prefab? It doesn't have to be a conventional method as long as it works. Many thanks Georgie