@gunnersgold You have to be very careful to do this the right way so it's not seen as deliberate deprivation of assets.
I'm not an only child but my DB has basically washed his hands of DM (she can be difficult) - I get on very well with him and he wouldn't expect any money from DM's estate anyway, so it's easier.
Our situation is complicated - what we're doing is selling DM's house and getting a small mortgage to purchase the property and do the majority of the work. Then we're going to sell my current property and use the equity to finish off the things I want done on my side of the house. DM can live in my side of the house while they're building her annexe and we also can slowly start to move stuff in from here. I've got two autistic DC and I'm autistic myself, so none of us handle change very well. So this slow and steady approach will really help us three.
I was concerned that the house could potentially be sold in the future to fund care for DM - although the theory is that being right next door we should be able to manage most things at home. I was a carer for my dad up until his death so I know what it involves and I'm OK with it.We spoke to an estate planner and we've put the house in trust. Happy to give you more info about this if you want, but avoids issues when Mum passes and also protects my investment ie/capital from sale of my property. The way we've set it up also means it can't be sold and used for care fees (should they be needed).
I did a huge amount of research into the different options. To cut a long story short, it depends what type of accommodation your DM wants and how much room she needs. My DM originally was going to have a cabin in the garden, they're really lovely to be fair, I'd live in one! They don't all need planning permission, it depends on the size.
The other thing I looked for was a wide house - and they are really bloody hard to find. The idea was that with a wide house it would be possible to split it more easily to the side and then maybe add extra space to the back under permitted development, without needing planning permission.
As it happens, I found a house on a corner plot where they have done a side extension which feels entirely separate to the main house and was actually used as an annexe. So we're going to add some more space to that with an extension and also put in a full kitchen in the annexe. We're also adding soundproofing between the side annexe and the main house so there is some privacy. We are then also putting a 15ft x25ft extension on the main house to give us the room we need. It all needs planning permission but the architect says we won't have any problems. There's no reason for it to be refused, and we're just getting that done at the moment.
You can apply for planning permission before you own the property. That means that when you get the keys, you'll be almost ready to start building straight away. A good architect is really worth their weight in gold - our one knows I haven't got a clue about the process and has been brilliant in talking me through each step and answering the million and one stupid question I keep asking!
So yes, ideally you'll need to apply for planning before the purchase and you need to find somewhere that you can be confident the planning will be granted. The chances are that the planning decision will come through before the purchase is complete. Also, you can apply for outline planning first - that's a slimmed down process where you don't need to supply full plans, and it's quicker. But it will indicate whether full planning is likely to be granted.
Happy to answer all/any other questions either on here or via pm - for me, being next to DM will really help both her and me. I find it hard splitting my time to trek over to see her - I home educate the DC now and I work full-time (self-employed) and being able to fit in seeing her around all of that just by popping next door will be a bloody godsend. She feels very lonely at the moment so I think this will be really life-changing for her too.