Legal Question in Real Estate Law in California

Sure, documents of the kind you describe (lease-to-purchase, rent-to-own, etc. etc.) are used from time to time and certainly could be customized to accomplish what you're thinking of.

However, why bother? You say you'll be getting the home via your friend's trust when she passes away. This is probably better and simpler than using a lease-to-own, since property acquired by inheritance (i.e., by will or trust) receives a step-up in basis and you'll avoid a pile of capital-gains tax and perhaps some gift or inheritance tax. If you want use of the home in the meanwhile, sure, by all means lease it from your close friend -- but use a conventional lease, not one that appears to transfer ownership or any unusual rights.

Of course, there's some risk to you that if your documents don't force your friend to leave you the home, that she'll change her mind, or need to encumber or sell the property before she passes away. That's a risk you need to weigh, and look at it from your friend's viewpoint as well. What if she needs this asset, for example, to pay for health care?

Finally, whatever you do, remember that the tax aspects (federal and state capital gains, gift, inheritance and local property taxes) are a very important factor and can swing by tens or hundreds of thousands of dollars either way depending upon how you come into ownership. Get sound tax advice from a tax planner or CPA.