Your basement or cellar is a space you already have. If it’s wet or flooded, you may not wish to use it for storage, so why not put it to practical use, give yourself valuable extra space and increase the value of your home at the same time? Unlike a loft conversion, you already have access, so there’s no need to free up space for a staircase. Also, there is no need to build dormer windows, etc.
Additionally, because it’s space you already have you won’t need to extend out into your garden or driveway. Better still, you rarely need planning permission. And it can work out much cheaper, so you’re onto a winner all-round.