Hello, I am Keith, one of the experts on Just Answer, and happy to help you with your question.
'Here is the Gov UK guidance on the subject:
If your garden and grounds do not exceed half a hectare (which is a little
over 1 acre), you are entitled to relief for all of it.'
Thus if this limit is not exceeded then there will be no Capital Gains Tax (CGT) due on the gain.
There is further guidance:
'If your garden and grounds exceed half a hectare, you may not be entitled
to relief for all of it. The area for which you are entitled to relief is called
the permitted area. It consists of the area that is required for the reasonable
enjoyment of your dwelling house as a home. The size and character of your
dwelling house must be taken into account.
If your garden and grounds exceed half a hectare, and you have disposed of
all or part of the garden and grounds, you should:
• enter details of the disposal and gain on pages CG 1 and CG 2, and
• explain in the ‘Any other information’ box, box 37, on page CG 2 of the
Capital gains summary pages why you think, if appropriate, all or part is
exempt from (CGT).
We may ask for further details in these cases and the District Valuer [now the Valuation Office Agency, part of the HMRC empire] will be asked to determine the size and location of the permitted area.'
So you might, in view of the use made of the land over the last 40 years, gain an exemption to CGT anyway.
I do hope that my reply has been of some assistance.