New Build Foundation Problems

[Lexden]

We have a new build home on clay in North Shropshire.
Just before the outbreak of Covid-19 (before the end of the 2-year snagging period), I informed the builder that we had heard a very large crack. On inspection, we found a number of large stress cracks in the drywall in a bedroom where the wall is supported by a RSJ extending across two pillars. There was also evidence of a broken external brick and cracks in the brickwork.

Detailed investigation has had to wait until my wife came out of shielding. Removing a section of the drywall shows that the internal breeze blocks are also cracked and the window board has dropped at one side.

The RSJ has been checked and found to be correctly sized etc so this week the investigations have moved to digging test pits around the house. All the work has been under the direction of a Chartered Structural Engineer.

This morning I think that we have an answer.
It would seem that during construction, the builder has correctly sized the foundations in a wall that is located near a hedgerow. The problem is that the builder has run the deep foundations level for about 15ft and has then taken out the difference in the required foundation depths in one step in a 90 degree corner (which supports one end of the RSJ where the broken brick etc have been seen.) I am advised that under Building Regulations the change in foundation depth should have been done in a number of step changes to cope with different soil dynamics.

To date, I have not advised my house insurer of the ongoing investigations as nothing has actually been done to the property but I am struggling to know what to do if the structural engineer comes back with a requirement to underpin. I am told informally that underpinning is likely to be needed under the foundations where the two levels meet extending forwards and around a patio door that sits under and forward of the RSJ.

Clearly, this fault has nothing to do with subsistence or heave as such. The builder seems to accept that the onus is him to put things right but I do not want to end up with paying extra insurance premiums for what is a major building fault.

How best do I protect my interests? Grateful for any thoughts/advice.

[New Home Expert]

There are clear warranty standards for stepped foundations.
The height of a step should not exceed the thickness of the foundation.  NHBC Standard 4.3.8
The step should overlap by twice the step height, or foundation thickness (max 500mm) or 300mm whichever is the larger.

On a corner this is probably not possible.

Either the NHBC or your builder (who is it by the way?) has to deal with this under the warranty.
Regarding your insurance and disclosure to future buyers, this is a building defect and not as you say subsidence, so should not re occur unless there is more to this than you say.

[Lexden]

Thanks.
I now have the structural engineer’s report which has come up with a number of OPTIONAL recommendations including underpinning. The others are do nothing and put additional drainage in the garden. 

I have suggested to the builder, who is local to Shropshire, that the NHBC needs to concur his intended repairs and confirm that their Structural Defect Warranty will remain place. It is not up to me to decide which of the optional recommendations would suffice (as was suggested). As my buildings insurance is due for renewal next week, I have had to come clean with my insurer.
I fear that this going to get complicated and expensive.

[New Home Expert]

Out of your options presented, doing nothing is not an option.

Fixing drainage only may (or probably will not) stop the subsidence getting worse.
I reached my conclusions just from your photographs.
Quite why the structural engineer and the NHBC seem to be avoiding underpinning (which is required) I don't know.
Other than the cost exposure to the NHBC.

Expect your insurer to either decline cover completely or exclude subsidence from your policy.
Be aware that insurers  share risk information with each other so getting alternative cover from another insurer may be difficult too.