You can remove the bottom cladding board to get an idea of the timber structure underneath. A ledger board is then fixed to the existing timber of the house. The decking joists will affix to the ledger with joist hangers.
The cladding boards size might now need to be adjusted to accommodate the ledger and infill the gap remaining. The cladding board can now be fixed back in place with the addition of flashing and weatherproofing to suit.
Here's a D.I.Y. step-by-step tutorial on How to build a deck which you might find helpful in your project.
If you would like more information about the process or have other questions, please let me know.