There are a couple of houses that need painting every few years, and a
place in each house where a ladder won't quite reach.
I have a very solid 3.5 metre aluminium stepladder that almost
reaches. New longer ladders are extremely expensive.

I was think that if I made a couple of platforms I could put the
ladder on top to get the extra height of 600 to 900 mm.
Basically the platforms would be like a couple of big boxes that I
would make out of 100x50 or 150x50 mm timber, since I have a lot of
that lying around. I'd like to use plywood but I'd have to go and buy
it.

I'd have outriggers of 100x50 under the boxes to stop them tipping
over. I'd have a harness on when up the ladder, attached over the
roof.
Any thoughts, derision etc?