If the dimensions of the original fireplace are larger than your needs, a supporting concrete lintel will need to be put in. When you've successfully supported your fireplace with a new lintel it will then be possible to renovate or upgrade the hearth. You might discover that your existing hearth has been sunken into the floor, however to abide by latest regulations it is required to be raised above floor level. If your fireplace isn't of a standard size you may have to purchase a custom-built hearth, however there are a whole host of pre-made hearths in marble, slate or concrete.