So, to my initial problem. Where to build the Cobra?
I considered renting space nearby, but a big part of my motivation is to spend time with my son (and I don't want to
totally alienate the rest of my family), so it would be heaps better if I could work on it at home.

We have a double carport, but our local planning caveats prevent me closing it in, and anyway I like the carport how it is, plus I don't want to lose the existing car spaces!
I measured out the space behind my front fence (an 8' brick wall) and to the side of my carport. There is a good area that, once cleared and modified, would measure about 5.5m by 3.5m, so that is where I am focussing.
To do this I need to add a roll-a-door, remove the smaller existing woodshed, remove some oft he existing brick fence and gate, relocate the gas meter and water tap, level and concrete the ground, place a roof over the top, wire it up (for power!) and build one side wall.