I did exactly that Sunshine. Mine's 3m by 5m, wooden doors at each end. I didn't use thick enough water piping so I have had to add some extra wooden reinforcement to the frame. All in all I guess I paid around £140 on building mine, including the plastic sheeting which came in around the £70 mark via ebay. You can see on the pic below from a couple of weeks ago what I've done inside in terms of beds and planting.