Hi acarey01 Carey,
Please have a look at CG15180 onwards, relating to enhancement expenditure
CG15180 - Expenditure: enhancement expenditure).
It is up to you to choose how you obtain the market value.
If you are unsure of your valuation, HM Revenue and Customs (HMRC) can check it for you.
After you’ve disposed of the asset, complete a ‘Post-transaction valuation check’ form. Return it to the address on the form - allow at least 3 months for HMRC’s response.
Post transaction valuation checks for Capital Gains (CG34)
Thank you.