Stop water from coming into old garage conversion

Dunny1976

Hi all

Looking for some advice. We purchased a house (late 60's) with an old garage conversion which had damp problems. Our builders took it back to the brick and re did the conversion. They dug a trench calling the wall put it some plastic and filled with shingle. Two years later we discovered that water had been coming in (but under my step child's desk so not noticed). It only seems to get wet when there is heavy rain. I have removed some of the stones to see what the problem is and cannot see an obvious damp proof course. It is outside the builders guarantee period, We are not particularly handy ourselves and Cannot afford to spend thousands. Would be grateful for any advice? TIA

grumpy_codger

Is the water coming through the wall or the floor? Is the floor level higher than the paving outside?

Does the French drain (the trench) have any drainage arrangements? Otherwise it just partly protects the wall from heavy rain splashes. And the black membrane, how it is done, collects the water running down the wall and, possibly, traps it.