If you have a proper chimney and chimney stack (brick or stone affair with one or more chimney pots) then it will have a builder’s opening at the bottom (or should do as I have always found one).
The builder’s opening is also known as the “original fireplace”, “original opening” or “builder’s recess”. The builder’s opening is what the house builder provides. But very often it is too large for the householder and is filled in to create a much smaller opening – such as a small open fireplace or small recess for a gas/electric fire.
Almost all small open fireplaces have a larger builder’s opening hidden away behind.
You can excavate to a builder’s opening without touching anything to do with the houses structural integrity.
With two people this job can take anything between 2 hours and a full day (usually 2-4 hours in my experience).
The stove fitter’s job is to examine any existing recess, wall or chimney breast and decide if a builder’s opening is hidden away behind.
From this (small open fireplace):
To this (large builder’s opening):