As mentioned above, there is no CGT to pay on value uplifts on your principle private residence.
One way to do what you desire is to apply for and acquire Outline Planning Consent for the plot in the garden and sell the plot with planning permission. That way, the 'value enhancement' has occurred directly to your principle private residence and there is no CGT to pay. This may be an attractive solution, because most of the value enhancement will come from uplift in the value of the new plot with its planning consent (and not from the physical building you might add). The downside is that you have less direct control over what house eventually gets built there.
If you are taking it to that next step, you have to live in the new house at some point for a period of time. The revenue are not specific about this but they are wise to serial self-builders. You would have to develop the new property, sell the original one and move into the new one for perhaps a minimum of six months. What is important is that you give out no intentions that the plan was ever anything other than to develop the new house and move into it. So correspondence with mortgage companies etc. should reflect that as the plan. If, after a period of time, plans change that is fine. The revenue will not come after you for CGT on either properties.
If your current plot is bigger than 0.5 hectares (2.5 acres) there may well be a problem with the above address.
If you want to get into the technical side of things yourself the HMRC manuals are here. Happy reading.
http://www.hmrc.gov.uk/manuals/cg4manual/CG64200c.htm