We were advised by HMRC that to avoid CGT you have to demonstrate that the house you are selling is your primary residential address. There doesn't seem to be a fixed period over which you have to demonstrate this but the suggestion was 'about a year'. That said, the HMRC advisor I spoke to said that they were pretty pragmatic with regard to service personnel that had been living in MQs and had moved back into their own house and then had to move for civie job. Essentially they assessed on a case by case basis and are just out to ensure that the folks 'don't take the piss'!