Fire doors are required in specific locations in dwellings, most commonly on rooms opening onto a protected stairway in a house of three storeys or more, on the door between a house and an attached garage, and where an attic conversion creates an additional storey. Technical Guidance Document B sets out where they apply.
The purpose of a fire door in a house is to protect the escape route long enough for occupants to get out. Where the escape route depends on a stairway serving several storeys, doors from habitable rooms onto that stairway are required to have fire resistance, and self-closing devices may also be required depending on the situation.
An attic conversion frequently triggers these requirements because it adds a storey and lengthens the escape route. This is one of the main reasons attic conversions cost more than people expect, and it is a common issue found when a house with an unauthorised conversion is being sold.