Inheritance

68ComebackSpecial

If she spends a sizeable proportion of her cash on an annex and then gifts you the property she will still be assessed as if it was still hers should she ever need care. You would then either need to sell the property to fund the care or you and your husband would have to pay for it if you wish to keep the house - could you afford £50k, maybe more, a year?

Savvy_Sue

With granny flats, there's not only the separate Council Tax issue, there's also a potential problem that no-one but a close relative can live there. Tread carefully. 

TBagpuss

You / She would need to take advice. 

Depending on the size of her estate  Inheritance Tax might be payable on her death. 
If she were to need care then the house could be taken into account in assessing what help she might get for care home fees.

Her putting the house into your names would not help as it would be either a gift with reservation or deprivation of assets, if done to try to avoid tax / care fees, and neither would be in her best interests as she would be trying to save you and your spouse money by disadvantaging herself .

She can, and should make a will.

have or do you live for free in the house, or have you and her son been making financial contributions or carrying out work or otherwise improving the house? If you have, then you might (take proper advice) be able to have a share in the house transferred to you which would have the benefit of reducing her share and therefore the value of her estate / assets . 

It would of course also be open to you to buy a share of the house from her at market rates, which would give you some security as you'd you'd have an interest in the property, and she would have more cash which she could use if she needed care .