The advice I got from the DB counsellor I spoke to late last year suggested that I direct to my H those kinds of questions - how do you see this going, renovation, telling the kids, etc. Be ready to end the conversation and leave if you feel yourself getting upset - "thanks for giving me a lot to think about, we'll talk again later" - out the door. Don't argue, if you can bring yourself to do it, be supportive of his 'need' to move out. Try to treat him as you would a child that's planning to run away - just kind of nod along with the plan.
And then do NOTHING. You're right, this is not your idea and you don't have to participate. You might be able to find a way to say this calmly, but it may not be received calmly, so I'm not sure it's wise to be quite that upfront about it - maybe just kind of vague - oh, really, yes, well, I've been so busy, maybe I'll get to it tomorrow/next week, etc.
You can tell the girls that you've made the decision to do the reno because you need the money. Maybe the three of you can brainstorm some ways to increase their privacy - even something as simple as a curtain down the middle of the room could help. Acknowledge their disappointment and the fact that sometimes things happen that we don't like. Who knows what will happen over the next few months and whether the renos will ever actually start or finish?
