Can two fireplaces use the same chimney?
Smoke comes into your neighbour's house when you light a fire.
Smoke comes into your house or your neighbours house whenever you light a fire
There are 2 possible reasons for this:
1 - your chimney is compromised and you should line it if possible. Smoke and other dangerous fumes (C02) can leak out from cracks in the chimney. This may be accompanied by tar marks on the walls.
2 - the smoke is being drawn back into your house, or your neighbour's house, through other chimneys. This is often caused by the fire creating a negative pressure in the house and, as a result, sucking air down another chimney, bringing smoke with it. The best solution is to create an air vent, to feed air directly to the fire, and so remove that negative pressure. You can try temporarily blocking all the other chimneys, at the chimney pots if possible. If the smoke still appears then you know it is not this that is causing the problem. If you want to block off a chimney permanently then you need to use a top vent cowl, which stops rain etc, but lets the chimney ‘breathe’. You will also need a vent into the fireplace at the bottom of the redundant chimney. If you cannot block the pots off, then have a look at what is happening up there when the smoke is appearing in your, or your neighbour's house (if you can get up there safely).