We are planning to downsize shortly, and will need to sell the existing shanty first. Since we are contemplating a smaller home in a 55+ community, I assume it will be necessary to put some $s down to start that construction while our house is on the market, so I'm in the same boat as you with limited after tax $s for a down payment or progress type payments. Need to go see Wells Fargo in the next few weeks regarding a heloc or possibly a bridge loan.
Doing things today that others won't, to do things tomorrow that others can't. Of course I'm referring to workouts, not robbing banks.