Hello and welcome to the site. Thank you for your question.
1. If I sell the new property am I liable to capital gains tax.
You will be liable to income tax and NI Class 4 and not capital gains tax if you build a new property and then sell it. HMRC would view this as activity of a property developer and not property investor.
2. I we live in the new building and sell our existing home are we liable to capital gains.
If you were to move to the new building and make it your only and main residence and sell your existing home, any gain from sale of existing home would be exempt from capital gains tax as it would be covered by private residence relief.
3. If my son who lives with us moves into the new property are we still liable to capital gains.
Basically, you are liable to capital gains tax on any chargeable gain and gain from sale of this property being a second home would give rise to CG, irrespective of the fact your sons would be living in it...
4. If both buildings are not sold immediately building works finish but are lived in by us, is their a number of years, will tax still be payable.
If you moved from your current dwelling to the new property and made it your main residence and notified HMRC within two years of completion that the new property is your main residence then all the period it remains your main residence your gain would be covered by private residence relief. If you were to sell your present residence within 18 months of moving out, then the whole gain on your present residence would also be exempt from capital gains tax.
More information on private residence relief can be found here
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.