If the new, smaller house is indeed his de facto PPR then the hother house ceased to be his PPR on taking up residency in the new house. CGT may apply to the percentage of any gain which relates to the number of yrs owned less the number of years as PPR less last 12 months of ownership. If the house is sold within 12 months of moving it is still deemed the PPR for that last 12 months ( to allow time for a sale really ) and no CGT is due. However if someone moves out, keeps the house for another 5 years, then sells, there is an exposure there even if no income was generated from the property.