This scenario doesn't seem to be in the interests of your father. A straightforward purchase would be, and the money raised would be available to pay for your father's care. The route you are looking at is fraught with all kinds of difficulties. Firstly, the developers requiring an 18 months option. At almost 80yrs of age and suffering from dementia, not wishing to be pessimistic, but 18 months is a long time at his age and condition. Secondly, planning permission can take a lot longer than 18 months, and if refused the time taken for appeals would make it even longer, and after that the developer could decide not to go ahead. It' is not very likely that a re-mortgage would be possible in the given circumstances, as you would be re-mortgaging your father's property, not yours, even given your intention to purchase the property yourself. In the event of your father passing away during such an extended waiting time, and you not being able to obtain a buy-to-let mortgage, the house would still have to be sold to provide the other beneficiaries with their share of the proceeds. It appears that there are too many variables in your plan, and would suggest you obtain some sound legal advice before deciding to do anything. Good luck.