Is this an existing house? What kind of fireplace?

Yes it is an existing house. The fireplace is a mendota dxv45

Is it gas or wood burning?

Look at an I-Joist manufacturer's framing instructions online. You might be able to cantilever from the existing structure even if you have to reinforce a joist with an LVL beam but success will depend largely on the total weight of the enclosure walls. You will probably need the advice of a design professional or a manufacturer in order to get a building permit or at least to avoid a structural mistake.

You might also try the Remodeling or Fireplace forums.

I think you need a local expert who can look at your situation first hand.

I have a bumped out dxv 45. The space I bumped it into was a deck with 2x10 stringers and a 2x6 deck. No reinforcement was needed, even with cultured stone going up 24 feet.

The fire place isnt' that heavy--shipping weight is only 250 lbs., but if you want to face the inside or out with natural stone, the area will probably need re-inforcement. If the bump out is near ground level, it won't be hard to lay a small cement pad for the FP.

If you are inclined to finish it with tile or cultured stone I think there's a good possibility that a cantilever will work.

Side note: I'm having big issues with my 45. It just doesn't look right. The flames are too uniform, and barely peak up between the logs. My two mendota 42s are tremendous, everything I hoped for and more. I'm starting to suspect that the 45 has something wrong, like a pressure regulator that isn't letting in enough gas.

Oh, another thing to remember: for a good burn, you can't go straight out the back with the vent pipe [even though the pics on the website makes it look like you can]. Especially if you are at a higher elevation, the vent needs some vertical rise for a good 'draw' of air. Try and get a sense of how much experience your dealer has with these units, and judge his advice accordingly.

oruboris, Do you any pictures you could share? We are only finishing the outside of the bump out with vinyl siding so no additional weight will be added with stone, the bump out only needs to hold it's own weight and a fraction of the stove since a portion of the stove will be sitting on the current floor (5.5") of the stove anyway.

Make sure you frame out the rough openings according to the manufacturer's specs. This is what your local code enforcers go with on fireplace openings as well as the venting installs,mechanicals,etc. Do everything mandated by the manufacturer of your stove.

Concerning the structural framing of the bump out, i agree with the above advise to have a licensed structural in your area come over and take a look. Well worth a consultation fee.

Did you ever figure this out?