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Any habitable room opening onto the protected fire route (corridor/staircase) will need to be FD20 rated.
The trouble with the fire door thing is everyone props them open. I used to live in a 2001 built house with self closing fire doors that would take a toddlers fingers off which seemed a much bigger risk than a fire so they were wedged.
In my current place when we added the loft conversion the non-self closing doors were deemed adequate as long as we had many smoke alarms. Seemed a bit rubbish but we didn't want to replace the doors so didn't question it.
Any habitable room opening onto the protected fire route (corridor/staircase) will need to be FD20 rated. That includes your kitchen and reception room double doors/large opening, it also includes bedrooms on the upper floors.
An alternative to the above is a sprinkler/mist system throughout.
A smoke alarm is needed regardless of the above.
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Edit: The above is for works to an existing 2-storey building (ie: typical Victorian housing stock) where you are looking to do a dormer extension. Different buildings including taller buildings, new-builds, flats in larger houses etc all have different regs.