Fireplaces have been warming homes for centuries. When you wish to install or build a new fireplace, you need to take into account code requirements for fireplace surrounds. The fireplace surround can consist of a mantel, hearth extensions, or the parts of the hearth that extend in front and beyond the firebox, and the firebox itself. Since you are dealing with open fire in a fireplace, regulations must be strictly observed.
The mantel can be made of a combustible material like wood. However, if you do install such a mantel, it must be at least six inches away from the fireplace's opening. If the wooden mantel is within twelve inches of the fireplace's opening, it should not extend out more than one-eighth of an inch per every inch it sits from the fireplace's opening.
You can have a hearth extension as part of your fireplace surround. It must be made of masonry or concrete. These materials need to be supported by materials that are also non-combustible, and strong enough to bear the load. If necessary, you should reinforce these materials. Make sure as well that there's nothing combustible resting against the hearth or the underside of the hearth extension. Make the hearth at least four inches thick, and the hearth extension at least two inches thick. The only time you can have the hearth extension be thinner than two inches is when the opening of the firebox is raised above the hearth extension by eight inches. When this is the case, you can make the hearth extension 3/8 inches thick, as long as it's made of stone, masonry, tile, concrete or another non-combustible material.
The firebox needs to be made of non-combustible materials that are grouted or mortared together to make one solid unit. The materials can be brick, concrete or stone. You can line the firebox, but the lining needs to be eight inches on the back. When you use bricks, make sure the joints between them are not more than 1/4 of an inch. If you don't have a lining, make sure the walls of the firebox are 12 inches thick. When you use bricks, make sure they are firebricks.