That little bit of extra investment could spare you from buying something you don't want. 

Any remaining material or extra materialwill be your responsibility to return.

What extra steps, beyond what's typical, would you take to make sure this goes through ok?

On a unit where I require first, last and security (usually SFH's) I will sometimes allow payments on that "last month" (something like and extra $200/mo until it's paid) but nobody moves in without at least a full month's rent and equal amount of security deposit.

In this case if you had done this you would have gotten another $61,000 in payments for the last 96 months. (96 x 636)  Free money and a nice way to build a retirement income down the road with no extra expense out of your pocket.Great job anyway. 

Proceed with extra cautions. 

So far, I’m thinking about extra closets with locks, maximum number of bedrooms feasible (aiming for four or five), fireplace and some kind of a nice outdoor deck or screen porch, possibly a hot tub.

I want to use the extra money I have every month to invest.

Your contractor should have at least communicated this with you when before incurring the cost of the extra material, best practices is to provide you with a change order as an amendment to the contract.

ESPECIALLY in the Millions.... you MUST do an IRR and take EVERY Future Cash Flow into Account.I know that some readers will just be biased against the IRR, they will still be the horse that I bring to the Water but just refuse to drink.I say that if you are serious about expanding your portfolio, becoming that multi-millionaire (or billionaire), then do it from the very smallest to the largest investment as you grow.Caleb, if you are having difficulty with my posts and these spreadsheets above, you are welcome to go to your friend who understands the importance of the IRR.