I'd get rid of it. Apart from all the other reasons already given, unless it's crushed up small, there will be air gaps and the soil that you put on above it will wash down into the gaps and gradually subside leaving you with an uneven surface.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
If you can afford to get it shifted following local guidelines for waste management that's clearly the best option.
However, as others have said there are many Mediterranean style plants that would do well on a poor base and won't mind it if they're given a good planting start. My main concern would be the size of that patch relative to the rest of the garden and whether the contrast in planting style and growth would look odd and unbalanced.
You might even want to consider using some of it as a paved area for sitting or eating out and thus take advantage of your own ready made hardcore.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
I had misunderstood I think. I thought it was going to be all crushed up and buried elsewhere. It would be more viable if the pieces were more crushed up into almost like a hardcore, but there are chunks in there with multiple whole bricks still cemented together. If you lay topsoil over the top, there are going to be a lot of voids into which the soil is going to take a while to settle. It'll probably sort itself out and turn out OK but I'd want it crushed up some more, and the large pieces removed. Or you can have it all removed and new subsoil brought in, but at a cost.
"What is hateful to you, do not do to your neighbour".
I'd just get the builder to hire a skip and get him to remove it all. That's the normal way of doing things and he should have priced that into his quote. Sounds to me as though he wants to take the easy way out.
Thanks for everyone's feedback on this. The consensus seems to be to remove it all.
I can't see any value to me of burying the rubble, just a range of risks and longer term problems.
I thought it was just the base. Yeah, get the bulider to shift it. Way too much to bury - plus you'd end up with a big stockpile of muck from the hole you dug to bury it in.
When we had raised beds created, approx 2 feet deep, the bottom 12 inches were filled with rubble, much like the stuff in the photos. Over time the soil level fell by about 6 inches as the soil made its way down through the gaps. You will have the same problem with any rubble below ground which hasn't been crushed. Getting rid won't be cheap, but it's certainly the best long term solution.
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It all needs to go I think.
However, as others have said there are many Mediterranean style plants that would do well on a poor base and won't mind it if they're given a good planting start. My main concern would be the size of that patch relative to the rest of the garden and whether the contrast in planting style and growth would look odd and unbalanced.
You might even want to consider using some of it as a paved area for sitting or eating out and thus take advantage of your own ready made hardcore.
I can't see any value to me of burying the rubble, just a range of risks and longer term problems.