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A “builder’s opening” or “original fireplace”, is at the foot of almost every chimney flue and is of a pre-determined size (you get what you are given). Do not confuse a small open fire with the builder’s opening, small open fires are often built within the builder’s opening and excavation will reveal the larger recess.
You can sometimes work out the size of your builder’s opening prior to any excavation work but not always (more on this in “breakout”). It might be possible to go larger than the builder’s opening but you are then likely entering the territory of the experienced professional builder.
A house might have more than one builder’s opening. If a house has two pots then it will have two flues and two builder’s openings. If it has four pots it will have four flues and four builder’s openings (e.g. two downstairs and two upstairs). Builder’s openings, especially in bedrooms, are often bricked/boarded up and plastered over.
Fireplaces never share flues: each builder’s opening has its own flue all the way to its own pot.
Another two examples below: