If ground floor walls that are suffering from dampness problems because of high ground levels, it is often sufficient if no hygroscopic salts are present to lower the external ground level only without any replastering needed.. This will allow evaporation of any rising dampness at low level. A french drain can also be installed, these need to be drained away from the property and ideally need a rodding eye to make sure you can check and maintain it. I have seen many french drains cause more of a problem, if there not drained to a suitable soak away or drain they will create a moat around the property.
The below picture shows where we cut the concrete path to reduce the high ground levels to help combat the rising damp.
Finishing off with a golden gravel to make it more pleasing to the eye.
By reducing the ground levels we have allowed evaporation of rising dampness along with un blocking the sub floor ventilation. No damp proof course needed here.