Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
Is this has been her main and only home right up until the day she moves out then there is no capital gains tax, as any profit made is exempt under the private residence relief rules.
Link here re rules https://www.gov.uk/government/publications/private-residence-relief-hs283-self-assessment-helpsheet
The only consideration to be had is Inheritance tax as I would assume your mother would be gifting this amount - then were she to pass away within 7 years of making the payment - then all or some of it would need to be considered for Inheritance tax. Link here re gifts and the 7 year rules and what gets added back into the estate https://www.gov.uk/inheritance-tax/gifts
However if the value of her total assets are less than £325,000 then there will be NO Inheritance tax as her estate is below the NIL threshold.
The only other point I should raise is this - should the time come that she has to go into nursing care - then any application with assistance for care home fees would be refused on the basis that she deprived herself of capital - as she already nees 24 hour care - she would fall into this catagory
Let me know if I can assist further or if you wish me to expand on my answers
You are very welcome
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