OK here goes I hope to order steel in the next 2 weeks and its gonna be a 442 for fatlads like me.
So the questions are

Has anyone ever built on a board screwed to a wall. I was thinking about this to save room in the early stages as i still have a go kart to get rid of.
I was thinking of building it in sides from A1 back to K3 etc and then the front foot well from C to Q then the rear wall fromB1 to O with temp braces in for K3 and K4
Then build up the LA to LD section.
I could then make up right angle supports fixed to the wall board and brace them for strenght to support the already built sides. From there i could them make uo the floor etc and add the other side.
This would allow me enough room to start building as it would only encroach 2 feet into my shed area instead of 4 feet.

Any thoughts on this idea or is it a definite no no. If you think it would work should i fully weld each sub assy before final welding or tack only.

Apart from weight is there any disadvantage of using 25*25 for the whole job.

Yes, I think it could work, but I feel you'll spend a far bit of time dealing with all the supports to keep stuff from dropping off. The biggest issue will be when you want to test-fit a part, say, the rear axle. Maybe you've thought that through, that you'll build it such that you don't need to test fit anything. I OTOH was test fitting parts all the time, but then again I wasn't building a Locost. Tack welding is fine.

Was kinda hoping that the basic outer of thr frame was more or less going to be all that was built in this way, and that by that time i would have more room to do it on the flat. ie before doing the axle, tunnel and engine/box.

Don't see a problem with your proposed build sequence however there is an issue using 25mm x 25mm throughout.
Consider the W tubes for example, when you cut them at an angle the length of the cut is greater than 19mm, (they actually turn out pretty close to 25mm), this doesn't matter as they are welded to a 25mm member, if however they were 25mm to start with you would have an overhang to deal with. Apart from this type of joint I don't see a problem using 25mm throughout.
In the same vein I have replaced a lot of the 19mm round sections with 19mm square in my build.

Mick

cheers mix did you wonder where i got the idea of building the sides first?
Thanks