What are the requirements for fire doors in homes?
In England, the regulation is Approved Document B of the Building Regulations 2019

In Wales, Approved Document B of the Building Regulations (Wales) and amendments

In Scotland, Part 2 Building Standards technical handbook 2017

In Northern Ireland, Technical Booklets

In general fire doors are not required inside dwellings of one or two storeys. In a three storey dwelling (including loft conversion or basement) fire doors are required on doorways to the protected stair and corridor leading to the final exit. 

In some circumstances a sprinkler system may be allowed as an alternative to protect the final exit route instead of fire doors.