Defects devaluing house.

[New Home Expert]

I would need to see the construction drawings and the home to answer this.
As a guess perhaps you have a perimeter stone projection at this level?
More likely, a cavity closed for a cavity fire sock to limit fire spread.

[Bri]

I requested a copy of my house plans to which I was advised it's company policy not to hand them to the homeowners.

[New Home Expert]

Approved planning and Building Regulations drawings are a matter or public record.
The local authority will have the planning drawings, which may well be full working drawings.
Whoever did building control, probably the NHBC will have the approved BC plans which you should be able to see, but perhaps not have a copy of.

Be aware that in both cases, the drawings used for construction may well have been revised several times and those that could get produced in court may even be revised and backdated to help deflect the builder's liability.

[Bri]

Hmmm. I`ll try the LABC for a copy or a look. I bet this gable end was not designed this way.