I don't see how using the rental/sale income from Mum's house for her personal care could be called deprivation of assets at all. If you keep a paper trail of the care invoices, you should be well covered against accusations of that.
If your rebuilding plans mean that Mum's part of the house would be self-contained and you could stop her and her carers from accessing your part of the house while you were out, then that would work too.
Another thing that people with dementia still like to do is "tidy up". That means if she has access to your rooms and possessions while you're not there, you could find things get moved to different places with much resulting frustration looking for them.
What would worry me, about my own Mum, is hygiene! Knowing that she doesn't wipe herself properly on the loo, or wash her hands unless prompted, no way would I want her to be able to wander round my house touching anything and especially not doing any "tidying up" in my kitchen.
In that respect, a way for Mum or the carer to make drinks/snacks without having access to your own kitchen might be thought about?
Thank you Owley
Good point about the hygiene/kitchen etc We had not planned for Mums space to be a seperate unit. We are meeting the arthitect again next week so we can see whether he has any solutions for this potential problem. I am however already a fiend with the old anti bacterial cleaners so I will probably just go into overdrive..!