CGT rate for non-residential investment sold for housing

[MarkWilliams]

This is a question of interpretation of government gobbledegook on CGT guidance:

An industrial premises being sold by private individual with a significant (£1M+) capital gain over 20+ years of ownership. Originally intended for own business use but treated as investment income since 1995. It is unquestionably industrial and we can document this as far back as 1952 prior to which it was probably farmland.
The sale contract is conditional on receipt of satisfactory planning and will complete in a few weeks' from now since Planning Permission for a number of residential units has now been granted. At the point of completion it will still be an industrial site and nothing in the sale contract compels the buyer to carry out the planning consent. Indeed, it is not unknown for purchases like this never to be built out and held in land banking whilst retaining an income (albeit at a very poor ROI) in continued industrial use. The buyer is a major UK house building PLC

Guidance offered by HMRC in June 2016 tries to define what is considered 'residential':
"Provisions will also make clear that a residential property interest includes an interest in land that has at any time in the person’s ownership consisted of or included a dwelling and an interest in land subsisting under a contract for an off-plan purchase. Rules will set out how gains should be calculated in the case of mixed use properties. "


Can anyone shine any light on the meaning of "land subsisting under a contract for an off-plan purchase" ? My expectation is that this is to ensure gains are taxed on individual residential units that are purchased from a builder and sold on before practical completion of the building works.

The second sentence is also of interest in other contexts. Is anyone aware of whether any relevant "rules" have yet been issued?

Any colleagues had actual experience of this as yet?

Many thanks