You can do it either way. You could install the wood first and all the marble after. Or you can do the whole thing and put the wood over the marble. The second method will be more expensive snce you will need more marble. On the other hand, the installation of the marble will be easier since you won't have to cut around the wood. Either way you can install the wood surround and mantel using the correct construction adhesive (see label on product) or attach it with screws (or even use both). My preference is to use at least some screws. If you use screws, get some lead anchors and drill into the brick/marble with a masonry bit sized to match the lead anchors. The screws then go into the lead anchors and will hold fast. Also, make sure that the clearance from the wood to the fireplace opening meets the minimums prescribed by your local building code!

I've torn out my heat emitting gas insert and am rebuilding firebox to take either wood or gas (like it used to be). It will be mainly for decorative purposes since there will be a TV above, but I plan to burn wood in it at least once to get a used look to the gorgeous sand herringbone I'm putting in. My contractor thinks 8" is enough space between wood mantle and firebox. I just got back from shopping for wood mantels (surrounds) where they said 12" is needed. Anyone know? From this photo it looks like the surround doesn't have to be big...small in fact. I will mostly burn gas https://www.houzz.com/photos/park-slope-brownstone-traditional-living-room-new-york-phvw-vp~368902-Herzog-eclectic-living-room-new-york I had to take the old firebox out since it had so many heatilator vents that would heat up the TV mounted above. So the only heat will come out the front. I do have a chimnney. Thanks

What temperature zone do you live? If it is cold climate, you might be better off replacing the insert with a new one that looks better. Is the plan to burn wood? I live in VA. In my prior house I had a real fireplace but had to put glass doors on it due to drafts and heat loss. In the new house, I have a Fireplace Xtrodinare wood burner insert with glass doors. Aside from looking great, this thing works as my backup heater and has no drafts - I love it. If you are in a moderate to warm climate then you probably can get away without the insert. But before you pull it out, you should make sure that the hearth, mantle, and flue meet modern fire codes for an open wood burner.

I know you said that you aren't changing the carpet, but might you consider it? Only because, I'm assuming that is a gas fireplace, there is no need for a large tile area in front. You can go for a narrow hearth or no hearth. If you wanted to do that, unless you have a large scrap or can match it, you're stuck with the large hearth. I like the idea of doing built-ins. As they would be asymmetrical anyway due to window placement, you may just want to build in on the right to accommodate TV and components and leave the left side as is. Not that I'm suggesting this style, but just to illustrate a tv next to fireplace built in, one side only. Something like this might work for you, but without the overmantel. Note that this tile surround is very similar to your hearth tile. If you like that look, you could get more tile, perhaps and then tie it together that way. [Eclectic Living Room[(https://www.houzz.com/photos/eclectic-living-room-ideas-phbr1-bp~t_718~s_2104) by Shallotte Fireplaces COASTROAD Hearth & Patio