You have to live in it for it to qualify as your principal private residence (on which no CGT is paid). If you are building the second one to live in and you are eventually selling the first one, you can own both for a period of time without being assessed for CGT on either if you time it right - the maximum period you can 'get away with' perfectly legally is just under three years of ownership of both. You need specific tax advice to ensure you get the timing right.
Other than the above, you cannot avoid the CGT - assuming property prices DO rise for this even to be a future issue for you.