A concrete stoop and steps can be a formidable obstacle when trying to build a deck. A massive concrete stoop will be difficult to remove. Most of the time you will need to break the concrete apart using a sledge hammer or jack hammer and haul the debris away. Some builders choose to build over the stoop instead.
The first step to building over a concrete stoop is to install a ledger board on both sides of the stoop. You will essentially box frame around the stoop by installing double joists on either side of the stoop and placing a double header beam in front of the stoop in between the double joists.
You can fill in the area above the stoop by installing a 2x6 ledger board with 2x6 joists running out to the header. The 2x6 joists should not come in contact with the concrete stoop. You may need to rip down the joists to provide clearance for frost heaving.