If you have lived at the property as your one and only private residence, then so far it is exempt from CGT when or if you sell it.
However, when you move to another house and use your tax exemption on your new private residence, the garage and land you have retained from the old property would be liable for CGT calculated on the difference in value when you separate it now........deducted from the price you sell it for, in the future.
Note - This would only apply if the gain is large enough to exceed your annual CGT exemption, at the time of sale. !