IHT for share ownership property

Dear Sir, My mother and myself jointly own a property (50/50 ownership) , I would like to know how to calculate IHT, is IHT only applies to her share of the property? Under what criterias/circumstances that only her share of the property requires IHT? Thank you,

Hi,
Inheritance Tax is a tax on the estate (the property, money and possessions) of someone who has died.  
If your mother died owning 50% of this asset when she dies, or within 7 years of her death, it would count towards the estates value for inheritance tax purposes.  
Please have a look at How Inheritance Tax works: thresholds, rules and allowances.
Thank you.