The Ministry of Housing, Communities and Local Government (MHCLG) issues circulars and guidance about new or altered Building Regulations Policy….

The latest being the; Application of building regulations where additional storeys are provided to existing buildings: circular 03/2020

Building Regulations 2010 – as amended
4. Each case must be considered on its own merits, but it is likely that where additional
storeys are added to an existing building, some work on the original part of the
building will be necessary. This may not always be set out in the approved documents
and building control bodies should consider carefully the application of the building
regulations to these projects.
5. The regulations pose a series of questions that must be considered, the first being
whether the work is deemed to be controlled and notifiable. We cannot envisage a
situation where the addition of new storeys to a building would not be notifiable
building work, so the following questions then arise;

https://www.gov.uk/government/collections/building-regulations-circulars