Reduce size of downstairs chimney breast

If the chimney breast has one flue from the ground floor fireplace then in theory the chimney breast could be made narrower. However it is entirely dependant on the size & route of the flue, to be absolutely sure I recommend you contact a Structural Surveyor to advise. You would also need to contact Building Control to pay for a Building Notice and have the work inspected if you go ahead. At the satisfactory completion of the work your Building Inspector would give you a Certificate of Satisfactory Completion which should be kept safe as it would be required if you sold the property.

The Back Boiler should not be in operation if the Chimney is Blocked up? You only need to use a Gas Safe Engineer who will bypass Building Control and certainly do not require a Surveyor. We are Gas Safe Registered and can self certify the work as can all Gas Safe Engineers. You can alter the Chimney & Breast as long as the type of appliance you are going to install has the correct size airway to safely remove any products of combustion. If you are making the Chimney redundant, then it will need to be vented to avoid condensation. So in short terms, the outside area is not the part that matters, its the area in the Chimney if reusing for a Gas Appliance. With the Back Boiler it will have a Flue Liner and the are will be 150mm2 throughout its length. All depends on what you are trying to achieve with the Breast. re use as Gas Appliance or make it redundant. . .

Yes this is possible, I did the very same thing in my own property over 20 years ago, you just need to Cornell the courses out to the existing chimney breast at first floor level.