Dear safelincs,
Thanks so much for your help up till now. So following your advice, I asked Building Control what legislation they were using to advise that we needed an F60 door and they sent this doc:
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/832631/Approved_Document_B__fire_safety__volume_1_-_2019_edition.pdf
I have tried to digest this and I found their definition of "building" in the glossary is a bit starnge: "Any permanent or temporary building but not any other kind of structure or erection. A reference to a building includes a reference to part of a building", so I guess that my two flats can be separate buildings (?!) . If that is true, in the box of page 136, it says
"In a compartment wall separating buildings: Same as for the wall in which the door is fitted, but a minimum of 60 minutes
In a compartment wall: a. if it separates a flat from a space in common use E 30"
However, earlier on in the document, it says:
6.6 In a converted building with a maximum of three storeys, a minimum REI 30 fire resistance could be accepted for elements of structure if the means of escape conform to the provisions of Section 3 (page 19 onwards).
It is really hard to understand this process for a lay person, I really value your advice. Thanks for your patience.
Best wishes
Di