Forgive me but I'm going to play Devils advocate here and ask : would an outsider consider these plans unreasonable? Sisters A and B have decided that their mother should live with sister A in a separate, purpose built, granny annexe in a secure environment in the back garden. They haven't involved sister C because they all know they don't get on. Mum's house will be sold to finance this. Why would sister B agree to this if the intention was to misappropriate any assets? What happens when their mum no longer lives there? It's not a proper extension so is not actually adding value to Sister A's property - they will either have to try and sell it on or pay to have it removed. I don't know the cost but it's probably less than a couple of years of care home fees. Is it in your mum's best interests? An outsider might consider it is, in the short/ medium term. Safe, secure, modern, and in very close proximity to a member of her family. Almost a text book solution. Far better than a lot of the chaotic and unsafe lives a lot of our parents are living in that period between independence and permanent care. If/ when your mum needs to move to a care home, she will self-fund until her remaining assets have been depleted and then she will continue as before, financed by the state. How it funded is unlikely to make any difference to her day to day care so your mum's needs are not likely to be compromised. The only tangible difference is in any future inheritance for sisters A, B and C. A portion of it will have been used to buy the annexe, which I think we all agree will have depreciated in value and it unlikely to add much, if anything, in the way of value to sister A's property. Sister B doesn't seem to see that as a problem....and there lies, I think, the weakness in your argument. Without a viable alternative that your sisters will agree to, and that seems unlikely, I think there's a danger that this might be seen as a case of a continued feud with your sisters, rather than a discussion about your mum's best interests. If there are three siblings and two agree on a solution, I would have thought that any outsider is most likely to see their proposal as the best course of action. No offence intended here, by the way. Just putting a different slant on it PS. I'd be hopping mad if I was Sister A's neighbour and there was no need for planning permission, and something like that was built over the fence.