Let's assume your $15K estimate to have an external entrance to your basement is accurate.
I'm envisioning a doorway with a path leading up to it. For $15K the path for the doorway should look something like this photo.
(In case you missed it, I think $15K for a doorway and a path to be pretty darn high...)
But, again, assuming it's accurate.
You're expecting $12,500 to $15,000 a year in rent? And you won't have any extra property oriented expense since you're already paying the taxes, insurance, mortgage, etc. on it. Unless you have separate utility meters your utilities will go up, though.
That's an 80% to 100% return on investment in the first year. Damn. That's sweet.
For that kind of money, I would proceed with a plan to get that basement rented out.
(I would also try to find a way to get the work done less expensively. Look into doing some of it yourself. Youtube is your friend. So are some handy friends. What could you trade for their assistance?)
Every year you continue to work after your initial expenses are recouped puts you that much closer to your FI date if you invest those profits. And, of course, with half your living expenses covered most of the time, you would only need about half the stash.
Wi