How to calculate garden land value as a % of property value?

We had a residential property left to four family members in a trust, a fifth beneficiary was the sole resident who was allowed to stay in the property indefinitely. She recently passed away, ending the trust and we have now sold the house and seperately a section of the garden to a neighbour, but are unsure how to calculate the value of the gain on the garden land for CGT? For example, If we received the property at the market value of £300K (with garden land intact) and later sold it at £315K (without the garden land), and at the same time sold the garden land separately for £50K, how would we calculate the market value of the land at the time we received the property, to calculate the gain? The estate agent who sold the property estimated the larger garden would have added only an extra £10K to the sales price. Is there guidance for calculating the initial garden land value as a % of the property value? Would this be based on just X % of the property value, or dependant on the % of the plot size, or just simply the difference in expected value as the estate agent would suggest? Many thanks.

Hi MT023,
You will need to make a 'Just and Reasonable' apportionment to calculate the MV of the land at the time it was acquired. It is up to you if you get professional advice on this but if enquired into you need to demonstrate that 'Just and Reasonable' calculation.
You can also request a Post Transaction Valuation Check (PTVC) on form CG34 if the asset has been disposed of and a return is yet to be submitted. There are time limits on sending this to use (no less than 2 months prior to the return due date) but they will need to provide a figure for the MV of the asset at a specific date to be reviewed by the Valuation Office Agency (VOA).
Thank you.