You can stop leasing whenever you want to, buy the company that spent a five digit sum of money on that thing in your backyard will want to get then money back first, which is unsurprising.
That doesn't surprise me, but the way a bank would do it is this: 1) loan you the capital to build the structure, 2) create a payment schedule, 3) try to collect on that payment if you miss it.
You could then declare bankruptcy if necessary, which means you'd keep the structure and have some sort of way out from under the debt.
If Rent the Backyard has an ownership stake in the structure, it sounds like they could continue leasing the unit even after you declared bankruptcy.
People definitely use traditional home equity loans to build ADUs but this can be quite risky. There are more full-stack providers (like one of our building partners https://node.eco/ who's headquartered in Washington State) which make the process a bit less nerve-wracking, but it's still a lot of time, effort, and risk.
You could then declare bankruptcy if necessary, which means you'd keep the structure and have some sort of way out from under the debt.
If Rent the Backyard has an ownership stake in the structure, it sounds like they could continue leasing the unit even after you declared bankruptcy.