@Cossy
I really feel for you. I understand about living with significant health issues as I do too, MS, arthritis, hypothyroidism to mention only a few.
I can see why you'd want to have some time andspace all to yourself but the way I interpreted your post was that you wanted to live alone, I may be wrong but that's how it came across to me and I apologise if I am wrong.
If I am right there are certain things that need serious consideration before taking that leap. I live alone and do sometimes have difficulty doing things, even just a simple no, I can't do it. It's time like these that I wish I had someone here, but I don't.
I've had falls, days and weeks when I couldn't walk and have been confined to my bed, I've even gone temporarily blind. How can you feed yourself when you can't get out of bed or can't see to be able to move around and prepare the food? (I know blind people can manage really well but if the blindness comes on fast with very little warning, it's difficult to manage).
Other posters have suggested a shed, conservatory, office, a room at your mum's and I believe this is the way to go, along with making sure that EVERYONE pulls their weight. Draw up a rota if necessary.
Your family needs to understand what your health conditions entail and how they actually affect you. They need to do their bit with looking after dad and granny, you can't do it all in your own, and, they also have to look after you. You've looked after them whilst they've been growing up, now you need the help, it's your turn to be on the receiving end.
Basically what I'm saying is get everyone at home organised so that you can take some time out for you. It's clearly better, if you have significant health issues, to have someone around who can help. I can assure you that as much as living alone does have some benefits, there are an awful lot of pitfalls like no-one being there to pick you up if you fall, you have to be strong enough (physically and mentally) to be able to pick yourself up and get on with things on your own.
I implore you not to move out and into another home on your own, it really is difficult, just organise the family and you'll get some free time where you can shut yourself away or go out for a walk or something and you'll start to feel better about the whole situation.