Conservatory Ideas please?

We can't afford an extension nor do we want to move. My wife wants some more space downstairs and has suggested a conservatory. We have a timber deck at the back of the house about 3.5 m X 4.5m which returns along the the side of the house.( pretty typical for this area) I have noticed some cheap but decent conservatories coming in at less than 3K which could be assembled on the timber decking. Some message boards suggest they can only realistically be used for about 5 months of the year. It seems possible to lay heavy rubber flooring on the decking to help keep out the draft. Budget means no substantial heating can be installed other than mobile. The budget is about 3K so DIY is the only option. Is it worth it?