Wildly varying DPC levels with shallow foundation for patio build

29 Aug 2022
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United Kingdom
Bit of a uncommon DPC/foundation situation here has left us in a pickle. I've highlighted the DPC levels in green. See Picture.

Wall Panorama3.jpg

We bought this house 2 years ago. We found its a 1970s, had an extension in 1975 and a large interior redesign in the last 15 years that included moving the rear doorway. This rear section is single story cavity wall.
The left side of the picture from the right edge of the door is the extension, the middle is where the old doorway was, and the right side was original. There are no damp problems in the house.

We had to rebuild a soakaway because the previous failed (stone filled hole, no membrane, FAR too close to house). So we opted to also excavate for a patio. Found and removed several tonnes of previous construction/renovation waste in the process.

The DPC levels vary wildly across the back, but to make matters worse it's only 35mm higher than the concrete side path on the right side AND it's the absolute minimum height above of only 3 brick courses above the foundation on the right quarter (allowing 225mm cavity clearance for building regs).
To be clear the foundation is just below the bottom visible brick on the right quarter, 3 bricks below the DPC. Furthermore the garden slopes upwards from the house (were adding a french drain at between the patio/garden in case), so higher patio would be prefered.
The foundation extends about 150mm away from the wall. This all makes it tricky to get any kind of drainage channel or slope in there. We believe the foundations are so wide/deep because clay soil around here.

This means a patio almost can't be 150mm below the DPC all the way as it would literally be cemented to the foundation. It would be too low anyway vs the path and the much higher garden, while making it a pit notably below all surrounding ground levels (esp. neighbors).

We are very strongly considering a lean-to canopy over almost all the rear wall to remove almost all the water getting there (piped directly to soakaway).
We would leave a ~100mm air/stone filled gap to the wall and either pipework, membrane wrapped stone, or proprietary solution to ensure the small amount of water can drain to the ground under the patio subbase.

Our options were considering (left side to right):
A) 150mm Below door DPC, slight slope to level with DPC/concrete on the right corner.
B) 150mm Below door DPC, step in middle to 75mm below DPC on the right, small step up to side path.
C) 85mm Below door DPC, step in middle to level with DPC/concrete on the right
D) 85mm Below door DPC, NO step in middle, ~75mm ABOVE DPC on the right. ~100mm step down to side path.

None are ideal, but our prefered solution is D with Canopy + 100mm air gap + solid provisions to ensure gap drains to soil below patio (i.e. so it doesn't just become a solid water collecting channel on foundation).

How would you deal with the combined DPC and foundation issue for this patio?
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