tell80,
Is the house on a hillside?
It would be unusual for a 1930's - 1950's semi to have solid walls.
No, not on a hill. A couple of streets away the road rises to join the main road but in the other direction it's all pretty flat. It's definitely solid walls because my Dad tried to get cavity wall insulation.
The interior right rear corner shows rising damp and higher up what looks like penetrating damp.
To show more: Interior left and right rear corners could do with the wall paper stripped off, & skirtings removed.
The rear air bricks appear to be almost clogged up with some material - bitumen?
Best practice is to have air bricks every 1.5m to 2m on accessible walls around the house - you show one air brick at the front?
Your DPC should be in the mortar bed below the joists - perhaps the air bricks are sitting on it?
The rear steps are bridging your DPC.
The sub-area joisting below any upper wall damage needs inspecting - this is done by crawling under the floors or lifting floor boards.
I'll get him to check the air bricks to make sure they're not clogged. I think the only other airbrick at the front is the one in the porch.
How would he prevent the rear steps bridging the DPC? I've been thinking that it would be good to raise the level of the patio to be level with the house and then have a slope or step down to the grass, as it would be safer for my parents as they get older to avoid the risk of them tripping onto the decking as they step out (tripping on to the grass will be less dangerous). Obviously I don't mean building it up with concrete as that would block off the air bricks but maybe it would be possible to raise the decking up on stilts (removing the steps first) whilst keeping adequate ventilation to the airbricks.
The render either side of the patio doors has minor, vertical cracks, possibly caused when the doors were installed.
Render should be installed with BellCasts - see google.
We dont even know what the white paint is so I see no reason for removing the paint (or render,) except maybe for minor repairs? The black stuff could be bitumen paint?"
The patio doors were actually replaced a couple of months ago, so the cracks might be from then.
Apparently the render was only fitted a few years ago, so hopefully the contractors would have used the right stuff. The walls were pebble dashed maybe 35 years ago, when the utility room next to the kitchen and behind the garage was added I think. I'll see if I can find any old photos which might show that the black paint was already there back then. As my Dad is pretty sure the DPC is bitumen, I wouldn't be surprised if its bitumen paint.
Testing drains for leaks is a simple matter for any DIY'er.
He says he had the drains checked a few years ago and they were fine but I'll see if he can find out exactly when. I doubt he knows how to check them himself or he would have done so before and he's getting a bit old to be crawling around on his hands and knees now.
There are other issues but without a mental map of your house plan it's difficult to comment.
As suggested above, it might be best for you to get a RICS in for a survey of whats wrong and whats to be done about it.
Yeah, I'll get him to look into that. A full Level 3 Building Survey would be a bit expensive at around £1,000 but I understand a specific damp survey only costs £100-£300, so hopefully that will be sufficient.