When we moved we put on a conservatory about 5m x 4m wide. White plastic. This was 3 years ago. I bought the basic kit from Wickes for £6k-ish and paid a pal £3k to put it up with me buying materials say another £3k (it adds up!). Concrete, decent bricks for dwarf walls, ceramic tiles for floor, lights, drainage, electrics etc.
If you are a competent d-i-y er then you can save quite a bit. If a company does the lot then you can easily get charged £20k and it will probably leak.
Mine could only be located on a South wall so it does get too hot even with air con on. dl