Taken from a previous tech bulletin TB150 issued by corgi:
British Standard (BS) 5440-1: 2000 - Installation and maintenance of flues and ventilation for gas
appliances of rated input not exceeding 70kW net (1st, 2nd and 3rd family gases) – Part 1:
Specification for the installation and maintenance of flues – states: “terminals shall not be sited in
covered passageways between buildings”.
Therefore it is not acceptable to position a new flue terminal within the confines of a covered
passageway. If an existing appliance is to be replaced, and the flue currently terminates within a
covered passageway, an alternative location for the termination will need to be sought. This may lead to
the appliance having to be relocated.
You need to have a proper assessment by a gas-safe registered engineer. One would hope all is well in new build!