I am working on a renovation of a Victorian house which is split into two flats (sketch attached). There is a small entrance lobby shared by the Ground Floor (GF) and First Floor (1F) flats. Both doors to the flats are fire rated with smoke seals. The 1F flat is under 4.5m  above ground level and it doesn't have a loft. The GF flat is open plan and each room has a direct escape to the outside via a window. As part of the renovation, the approved inspector is asking us to introduce an internal protected hallway in the GF flat by putting a door is the corridor. Why is this necessary if we already have a protected lobby between the two flats? I had an identical project signed off in January 2020 without the additional door. Have the regulations changed since then? Where in document B1 is this explained? 

I agree with you I cannot see why the AI is asking for it, I would be more concerned about the bedroom off the kitchen/living/dining room which appears to be an habitable inner room?

They are applying "strict rules of ADB" to paraphrase the film Goldfinger - which says that every flat should have rooms accessed off an internal protected lobby (the common area it opens off is immaterial) a principle that's been around for decades.

Oddly they are ignoring the bedroom off the kitchen (unless they are counting an escape window in the bedroom as an alternate escape as it's ground floor).

They are acting correctly - of course ADB is just a suggested method of complying with Building Regulations and if you can provide an alternative approach that provides at least equivalent safety then it is acceptable too.

If you had a different AI who accepted a non ADB layout without compensatory measures then that was good fortune on your part in that case.