I would say that the likelihood is that they may not have Installed a damp proof membrane under the floor connecting the main house to the porch/patio.
The best way to approach it is to break out the concrete in the porch Down to say 600-800mm, you will probably have to break out 200-300mm of concrete, then earth or clay. You can then see if any damp proofing has been used.
Your options are to completely break out all the concrete, dig out the earth, down to say 800mm, install a good DPM and then pour concrete all the way back up.
Earth or soil retains the water, concrete is a better alternative.
If it's gone too far, you will need a company to inject a damp proof treatment in the wall costing a lot