Does this happen only to the wall that is very close to the perimeter fence or on all the walls? YES, IT AFFECTD ONLY THE WALL THAT IS VERY CLOSE TO THE DWARF PERIMTER FENCE. ATTACHED IS THE IMAGE OF THE AFFECTED WALLS FOR YOUR PERUSE
if it is on all the walls then the foundation was not built properly, my guess is that you did not do the proper German floor. IT'S NOT DAMPNESS AS A RESULT OF DEFECT IN DPM OR DPC BUT RATHER WATER ACCUMULATING WITHIN THE HOLLOWS OF THE SANDCRETE BLOCKS (OF THE DWARF FENCE) WHICH IS PARALLEL WITH THE SITTING ROOM WALL

Thank you for your prompt response.

One of my sitting room wall is allign with a dwarf existing perimeter fencing such that a thin space (2 inches) was created in-between the wall. Of course, my sitting room is higher than the dwarf perimeter fence hence external plastering was done to prevent weather effect. But each raining season, the interior surface finishing of my sitting room get moist, develop fungi and peel off surface finishing overtime. I had to repaint the surfaces at the end of each raining season. I've filled the in-between wall spaces with broken block & mortar but yet it didn't stopped the moisture on the interior surface. Can cement screeding of the interior surface be a solution? Or is there any water repellant paint that you can recommend? Please, I need your professional advice and thank you in anticipation of your kind response.