As you are planning to remain in the upper flat, there will be no CGT to pay on this as it will remain your PPR.
On the basement flat there would be a liability equivalent to:
The sales price
less half costs of conversion
less specific costs of equipping new flat
less fair value of the basement prior to conversion.
The last bit is the more interesting. Basically you could take the value pre-split and divide by say floor space or any other reasonable split. Obviously the bigger the value, the lower the tax.
You would also get your personal allowance of £7900, and if you have a partner in joint ownership, their personal allowance to consider.
If you need some help filling in your CGT form or estimating your liability for a small fee, please get in contact. I take it you will also need some legal help for dividing the property. I know a very competent and reasonably priced solicitor you can use if you are interested.
James Smith
Chartered Accountant
www.uktaxshop.co.uk
01284 764436