Fire doors arrive in a building through building control and are then maintained under fire safety law. Those are two different regimes with two different tests, and most confusion about fire doors comes from applying one where the other governs. Building regulations decide what has to be installed when work is done. The fire safety order decides what has to be kept working afterwards.
What the building regulations require
The regulations require adequate provision for early warning and for escape, and for limiting the internal spread of fire. Approved Document B is the statutory guidance showing one way of meeting that, and it is where door requirements are set out by building type: which openings need a fire door, what fire resistance period applies, where self closing is needed and where smoke control is required in addition to integrity.
Where the periods come from
The required fire resistance follows the element the door sits in. A door in a wall required to hold for a given period is specified to a related period, and in some locations to a lower one where the guidance accepts it. That is why a flat entrance door and an internal door to a garage and a door onto a protected stairway can all be fire doors and not all be the same door.
Handover, and the information that has to come with it
When work is completed, fire safety information must be given to the responsible person. That information is what allows the building to be operated and maintained as it was designed, and its absence is the single most common reason a duty holder cannot say what rating the doors in their building hold. Ask for it at handover, because reconstructing it later means opening doors up.
Where the order takes over
Once the building is occupied, the fire safety order requires the responsible person to maintain the fire safety measures in efficient working order and in good repair. A door installed correctly under building regulations and then propped open, painted over its seals or fitted with a lock that defeats the closer has stopped being a fire door in the sense that matters.
Questions people ask about building regulations for fire doors
Do building regulations apply to replacing one door?
Replacing a fire door in an existing building is generally work that has to meet the requirement for that element, so a like for like certified replacement is the safe course. Downgrading a door because the original looked ordinary is not.
Does an old building have to be brought up to current guidance?
Not by the building regulations, which apply to work when it is done. The fire safety order applies continuously and asks whether the measures are adequate for the risk now, which sometimes reaches the same answer by a different route.
Who enforces which?
Building control for the work, and the fire and rescue authority for the occupied building. Both can look at the same door and ask different questions about it.