Mags - "so are you saying that if I bought a house (not primary residence) to renovate, with no intention of living in it,.............. no CGT to pay"
Not quite - one has to actually live in the second property for a period of time (and demonstrate that one has done so by, for example, getting one's post delivered there, paying CT and utility bills).
It's called playing the system.
"If the property market jumps up then it jumps up on both properties so I can see no reason to tell the HMRC....."
Fine except that, if there was a capital gain on the 2nd property, your suggestion of not informing could be tax evasion, as distinct from tax avoidance.
The purpose of the 'three year rule' to which I am referring was originally introduced to take account of what you are highlighting - that sometimes, with the best will in the world, householders end up with two properties for a period, having not sold one before buying another.
There's loads of stuff on it here: -
http://www.hmrc.gov.u...y/sell-own-home.htm#1
The bit I'm referring to is:
"The final three years (36 months) always qualify for relief. This applies even if you weren't living there during the final three years. It must have been your only or main home at some point during the time that you've owned it."