Hello and welcome to the site. Thank you for your question.
Your cost price will be made up of
- 50% share of the cost when you bought it in 1999
- the valuation of your mother's share that was transferred to you three years ago.
From the gain (sale proceeds less cost price as caluculated above).
- you can also claim for any capital improvements to the property during your period of ownership.
- costs associated with buying and selling the property.
Your reliefs/exemptions will be
- private residence relief for the period you lived in the property as your main residence
- additional relief for final 36 months (up to 5 Apr 2014) and the reduced to 18 months (from 6 Apr 2014)
- letting relief up to max £40k
All these reliefs would be available provided the property was your main residence at some point during the period of ownership.
Once you have taken the above reliefs into account, if there is a balance left then you claim Gains annual allowance against it (£10,900 in current tax year) and any balalnce left over would be taxed at 28% as you are higher rate tax payer.
The above explains how CGT would be calculated.
More information on private residence relief can be found on HS283 here
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.