I'd avoid borrowing from any sort of retirement vehicle. While you do pay yourself back, you miss out on growth during that time.
I can't speak on the leave, but if you do make this move, make sure you get in writing from your HR department that they would not call the loan due in that case.
Finally, is this a few small to medium projects or one large $40k job? Are you hiring the work out or is the $40k just materials?
We've done some medium to large renovations in the past by breaking them into smaller steps, paying for the materials out of our cash flow, and doing the work ourselves. If you're reasonably handy, take your time, and hire out only the parts you have to due to lack of skill, you can save a boatload of money.