HMRC do not provide a formal definition of what constitutes re-occupation of a property, but they look at a few factors to assess the ‘quality’ of occupation.
If you moved back into the property with it already on the market and sold it, say, a month later, HMRC are unlikely to view this as reoccupied.
However, in addition to the length of time you live in the property following ‘reoccupation’, HMRC will also consider what you use as your correspondence address, the location of any furniture/ household goods, whether you are on the electoral register at the property, named on utility bills and listed as living there for council tax purposes. They will also consider whether your family are also living at the property and where any children attend school.
We would always recommend seeking professional advice if you sell a property which you have not occupied for the entire period of ownership.