Incidentally, that raises one question. Let's say DCs live in caravan on site. Presumably you have to separate site of caravan from site of build. HSE prohibiting children on construction sites and all that.
Who is at work and since when did a family dwelling become a workplace covered by the Health and Safety at Work Act 1974? By that analogy, any maintenance being carried out on your home would render it uninhabitable...I think the Health and Safety Executive might be a bit surprised to hear about this extension to their remit, and property owners to the fact that responsibility for their own property has been taken over by whatever contractor is on site at the time! Don't actually have kids yet (we didn't have pro-conception expert Kevin McCloud on hand...) but have friends who are currently self building with 3, and they have the kids doing quite a bit of stuff.
I think the drawing thing is the bit I'd enjoy the most. The physical interface with Building Control is something I'd expect a layman to have a problem with. I've done it for years and I still thing they're AH bureaucrats. And the programming and establishing of the build sequence is the thing I think I'd stumble with. Putting in the drainage and the founds and the groundwater stuff before the first lift of brickwork and realising you had to go back and re-excavate, or getting your first fix electrics and plumbing right, before fitting the plasterboard and then having to take it apart if you got it wrong. And a lot of people don't get drainage and plumbing.
The drawings were actually very easy, as long as you have an eye for detail and accurate measurement. I found it quite easy to envisage exactly what I wanted and put it on paper, (much easier than talking to an architect intent on pushing yet another cathedral height windowed reception area. It was very time consuming though, and the architectural technician (who wasn't expensive) informed us that local authorities just won't grant pp to architects/ATs they don't know, no matter how good the plans are. With the barn conversion, again I found it logical enough to work out what rooms would go where, what would work, where would need more light, etc..
The groundworks and drainage are about the easiest part of it! You just make sure you get it right! DH is an electronics engineer so did his own wiring, he would much rather he did it himself because some of the work he has seen in past properties done by supposed "electricians" is just scary. Again though, despite having a degree in electronics, he had to get it signed off by a "qualified electrician", who admitted DH had far greater knowledge and practical skills than he had ever seen. Its so stupid - DH isn't even technically allowed to change the facing on a socket in one of our walls since the latest edition of the Building Regulations.
Ditto with the plumbing and central heating; at least we know the joints are soldered properly and not by some teenager on NMW who can't wait to get home - sorry but again, seen it too often before.
What worked for us was getting first one room habitable, as in wind and watertight, with a temporary electricity supply (some isolated circuit on a star or something). We then expanded. Actually I loved living in the caravan, we parked it in an outbuilding so it was undercover and kept some heat in.
The other thing about doing as much as possible yourself - it is amazing what odds and ends people sell off dirt cheap in Free Ads - that's where our ultra luxurious showers came from, ditto our taps, sinks, flooring, and much else. Its not so much economy of scale as attention to detail and time to apply it because its your own property, and the ability to recycle second hand stuff.
For the outer stuff, we were fairly confident we could master roofing and perhaps even brickwork eventually, but its so much faster to pay someone else to do it. And brickwork is tricky, so is getting a roof look perfect. We did harl the walls ourselves though (planners insisted on this finish), and painted it afterwards.
tbh though I might have a luxury 5 bedroom home now but I still spend the odd night in the caravan...