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 2×6 ledger board with 2×6 joists running out to the header. The 2×6 joists should not come in contact with the concrete stoop. You may need to rip down the joists to provide clearance for frost heaving.
Source: https://www.decks.com/how-to/483/building-a-deck-over-concrete-steps
