Main residence

oldbikebloke

my take on it is s223ZA is merely enacting what was previously extra statutory concession #D49 and all that has changed is instead of 12 months (with HMRC discretion to extend to 24), it is now a blanket 24 months

nothing has changed re conditions, the example under ESC 49 which states: "three circumstances in which you should allow relief for a period between the acquisition of land, including land on which a dwelling house stands, and the beginning of residence in a dwelling house on that site. "
https://www.gov.uk/hmrc-internal-manuals/capital-gains-manual/cg65003

Jimmo is of course right that occupation is an overriding requirement for PRR, but that is categorically "at some time during their period of ownership in order to qualify for relief. It is sometimes argued that private residence relief is due where an individual has acquired a dwelling house with the intention of making it their home, but for reasons outside their control they were forced to sell it without ever having occupied it. In these circumstances relief is not available; an intention to occupy is not enough."
https://www.gov.uk/hmrc-internal-manuals/capital-gains-manual/cg64465

To me, as explained under D49, it is implicit that "between" residence is not required from point of acquisition, eg does not begin until works are complete, so that expressly does not negate simultaneous PRR, provided occupation takes place at some time during the ownership of the new property. Then the new property qualifies for the 2 year relief and as such overlaps PPR on the old one.
In OP's case, we will take as a given the reality is occupation will take place before 24 months anyway, therefore effectively clarifying any question as to intent rather than actuality, as per the above quote. 

Jeremy535897

I wonder whether those who wrote the legislation actually realise that in many cases, you don't have a nice cut off point where you stop living in old house and start living in new house? When I took on a renovation project many years ago, I lived in the property I was doing up for 3 or 4 days at a time, and then went back to my "old" house to keep it maintained, wash all my clothes, and get away from the dust and grime. I don't know whether OP was in this position.