[TamSV]

Whatever she sells the house to you for, her Capital Gains will be calculated on market value. She can deduct the cost of any improvements to the property, but not repairs.

Having said that, property values aren't going to go up and may well come down, in the timeframe you're talking about and as mentioned above she will have an allowance (I thought it was about £10k now). If she's not intending to dispose of any other taxable assets that might well take care of any potential gain.

There is a possible issue with inheritance tax if she sells you a property way below market value and then passes away within a couple of years. Depends on the difference in value, the size of her estate etc.

EDIT: I'm assuming above that it's not your mums intention to live in the property.

BTW I'm not an accountant but had to take advice on CGT last year when my wife sold a house she'd been renting out for a few years.