Hi,
As there is no Capital Gains Tax liability arising from the transfer of assets between spouses and civil partners, you are considered to have acquired your wife's 50% at the value the share of the property was acquired for by your wife.
If you dispose of 50% of the property to someone who is not at arms length, you are required to use the market value of the asset when calculating Capital Gains Tax.
CG14541 - Consideration for disposal: market value rule: at arm's length
The difference between 50% of the market value at the time of disposal and 50% of the acquisition costs is either a gain or a loss.
There is a calculator below, to help you calculate any liability and provide guidance on reporting and paying Capital Gains Tax within 60 days of the completion date.
Tax when you sell property
Thank you.