Hi. I am aware that an internal office door can be secured by a cylinder mortice deadlock with a thumb turn on the office side. Can you point me to the relevant passage in Building Regulations please.

The office is small with only the one door to a corridor.

Thanking you 

AndrewMT

You won't find it as Building Regulations are functional and only give broad aims, as does the Fire Safety Order under which the relevant clause is:
(f)emergency doors must not be so locked or fastened that they cannot be easily and immediately opened by any person who may require to use them in an emergency;

A good fire risk assessment would identify that in the situation you describe an escape mortice deadlock would meet the requirements of Article 14 (2)(f) of the Order

As AB has said it is not in the building regulations but there is a clause in BS EN 179:2008 clause 4.1.2. Release Function, which says that the lock should release the exit door, from the inside, within 1s without the use of a key or similar device. Then you apply risk assessment and would a thumb turn meet that advice, which it would in most situations.

You must also understand fire safety is all about risk assessment and it is not prescriptive which means advice is not always in black and white.