Martin,
One easy way to achieve a custom fireplace opening (also check out page 249 of the reference manual).
1. Draw a slab (say 500mm thick) with the outline of the fireplace opening (arched head, round, whatever you desire etc.) with the bottom at 0 and the top at 500mm per above. Call up the slab settings and change the ID (under Listing & Labelling) to 'Wallniche'. This will be your void space.
2. Draw the fireplace box, mantle, stone surrounds, glass doors using slabs, roofs, walls or whatever. Make sure you apply the materials you want. Don't change the ID for these items!!!
3. Open everything in the 3D window and 'Save as Library Part (window)'.
You can then use this like a Window symbol and insert it into a 750mm wall and it will create the 500mm deep void and fire box described above.
Just keep in mind when constructing, that everything is 'face down' ie, the 'worms eye view' is the front view. Weird, but thats the way AC handles Doors & Windows. And make sure the front of your fireplace is set at 0 elevation and built up from there. And set the lower centrepoint at the project origin.
The above can be used to graphically create any number of 'Door/Window' custom openings AND the bits inside such as frames, doors, hardware. For a door/window just use 'Wallhole' in place of 'Wallniche'.
Cheers
Cameron Hestler, Architect
Archicad 27 / Mac Studio M1 Max - 32 GB / LG24" Monitors / 14.5 Sonoma