Hi 

my Grandma just moved into a new build house with an internal garage there is a fire door between the kitchen and the garage and then the ‘back door’ of the house is in the Garage.

The back door obviously has a lock but my grandma wants to put a lock on the internal fire door as well I have said she shouldn’t do this as it is a fire door but I wondered if she insisted if there are any implications for doing this I.e affecting insurance?  

I have read that fire doors can not be locked but unsure if this is relevant to a commercial property only  

There is nothing stopping you locking this door, although it would be advisable to have a lock that can be opened from the inside without a key to allow emergency escape. The fitting of the lock should not compromise the integrity of the fire door so should follow the door manufacturers instructions.