Self build timing the finance and CGT

[Raviroo]

Hi
I have Planning permission to build a home for myself and my family on some garden land at the said of my PPR (main home and nominated as such).
However, I can't afford to build it without selling my main home.
so the plan is to use savings to get the new home to watertight stage and then sell my main home. Whilst that sale is going through to completion, after exchange of contracts, then to finish the interior of the new build, paying at the end , when the new house is completed.
Then, move into the new house allowing the monies to be paid to the builder after completion. I think there will be about 6 week's to two month's difference in the gap between the sale completing on the old home and downloading the cash and then the builder completing the new home and then me paying him and living there.
I read somewhere that if you put a fence in-between the two homes -you are liable for CGT.

Can anyone help me understand if what I am doing -the order of events is still going to be free of CGT or anything else. The new home will be my main home forever, after that.

Or is there a better way of doing this?

Thanks

[LozaACCS]

The CGT liability derives from the gain arising on the sale of the original property, if the garden(s) formed part of the main residence and were within the permitted area (0.5 hectares), I cannot see a problem, perhaps you should provide more details on the associated land