Thanks for the advice. Great idea with the painting but I just think with the polycarb roof, the old fashioned window design I'd want to get rid of and options re: the floor and potentially underfloor heating I'd not really get what we would want and it really needs to be rebuilt at some point. I quite like the idea of dwarf walls as well and this obviously provides more options with plug sockets etc.

Sliding doors are proving very popular at the moment as they can stack to one side and not protrude into the garden space and be easier to move than full set of bi-folds so that would an approximate opening of about 4m with 3 x 1.250m (plus frame ) panel of glass that would all stack in parallel inline. Or you could try panoramic doors they are more versatile as they are detachable from each other and give you more options for you layout at the finish. There a tutorial on you tube ? Do you have building control on these works as I am finding that architects are making us use a lot more steel as in a goal post design for stability, depending on how wide you want to go with the opening?

I know this thread is slightly old now, but I've just used Bedec msp paint on the outside of my upvc porch and it's amazing stuff! Would highly recommend (took me years to pluck up the courage to try it) I spoke to the company's rep and got advice on what to use. Clean with an anti-fungal cleaner (NOT sugar soap as that leaves a residue) let dry. Scour with a green scoured pad OR a v v fine sandpaper which is only to take off the sheen and not to scratch the plastic. Wipe clean with water and let dry. Apply the paint with a brush not too thickly or thinly. Wait to dry 2-6 hours then repaint. Takes one month to cure. My experience - I did black over brown and 2 coats was fine. I found all the brushes I tried (not real hair ones) did leave some marks but I think that's due to the paint. Don't go over it too much in one spot. It's quite thick - plasticed when it dries so work quickly. Less of an issue inside when the sun isn't heating it up of course. Water based so cleans up easily and good range of colours. No primer required so v speedy job! I debated whether to try this or the bullseye 123 primer etc but I'm glad I went for this as it was a lot less work and it looks brilliant.

I believe important is also the nature of material this colour is on. If the existing kitchen window is wooden frame, painted white and the new bi fold door is aluminium anthracite/black I wouldn't mind. But if the white window is plastic white, this truly doesn't go together very well.