I get the pressure over Christmas, but the extension is for a loooong time, whereas Christmas is over in just a few days. If you can get it done well by putting on some time pressure, it is worth it - but it's not worth cutting corners. Can you have an outdoor tree instead dust? And perhaps a mini personal one each for the kids' rooms? I remember being a child while an extension went up, and you just sort of accept it far more readily actually than adults do - I'm sure in a house full of love they won't really notice/mind a bit of an "alternative methodology" this year. All that said, if an escape from the dust and bother is what is called for, your in laws sound great!
whatthefox - I think one thing that's stressing me is that so far I've been dealing with a builder who is really clued up - a really bright guy who is a great communicator, one of the new breed of tradesmen who has a degree, marking experience, the works. My electrician is much more traditional. He'd never seen a trimless recessed spotlight before and there has been harrumphing over the price (which I am paying!) and the faff of fitting them. I've been told I want fish in the pond next to the house (I don't) and I need a filter and a socket for this (I don't because I'm not having fish). I am struggling to get him to listen - I need his advice on whether I'm choosing the right type (there are lots of permutations), and I really don't get the impression he has properly looked at the data sheet I have given him. That kind of thing makes me nervous. 
It's the detail of fittings that is awkward. I have a flight of 3 steps from the dining room to the garden room, and I want some subtle ambient lighting for these that can be on all evening, to ensure that we are safe trotting up and down them. I would ideally like a recessed, trimless floor-washer that just casts a bit of light downwards. I found one by Astro, then discovered the it has an integrated bulb - and as it's plaster-in, that could be really annoying. Found an alternative one (the Tornado TF41), but it uses a weird specialist bulb, the Osram dragon eye), and I'm now going all around the houses trying to find out exactly how bright it is, and exactly how it lights so I can figure out exactly where it needs to be placed. I don't want to have to revert to more traditional wall lights, but I have to admit at this moment it would make life a great deal easier.