CGTax liability on my home if I buy a second property to renovate and move into before selling

We have lived in the house that we built 20 years ago without absence or sub letting. We are now considering downscaling, buying another property to renovate, move in on completion and then sell our existing home. The renovation could take up to 18 months, 2 years. Would we be liable for capital gains tax on selling our principle residence/home, as I would have owned the second property, but not lived in it, for 2 years? Also, as we built the property, would it be the total monies spent to build or is it just the land cost as a starting point to calculate the tax liability?

Hi ,
 

When you acquire a second or more properties, you will be required to write to HMRC and nominate which residence will be your main residence for private residence relief.  You have 2 years in which to do this, after which HMRC will decide for you, based on the facts of the case.  Please have a look at : CG64420 - Private residence relief: only or main residence: introduction: residence    onwards, which cover private residence relief. Thank you .