I would use 3 walls end to end - 200mm thick walls at the sides and 400mm thick wall in the middle.
With the same building material and surface materials they should blend together seamlessly.
Or you could do it as one wall using the polygon wall option.
But would that really be a 400mm thick solid concrete wall for the fireplace?
I would model it as it is built.
Barry.
One of the forum moderators.
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