NHBC Solo Warranty is designed especially for self-builders.
One important difference from their Buildmark policy is that there is no cover-everything 2-year, builder responsibility, warranty period .
There is no need to make a fresh claim and start all over again.
You have already notified the NHBC and the original problem has got worse.
Please do not spend money on surveyor's reports.
Write to the NHBC quoting your policy number and stating that you require the NHBC to investigate your claim. Be quick as your warranty only has a year left!
I would point out that render is not a structural element, but the fact it is cracking and coming away from the wall may be indicative of a structural issue which is obviously covered by the warranty.
However the render in itself is covered by your NHBC Solo Warranty.
Page 9 states: "What NHBC will pay for"
"A) The full Cost, if it is more than £1,000 (Indexed), of putting right any actual
physical Damage caused by a Defect in any of the following parts of the
Home or its Common Parts:
- foundations
- load-bearing walls
- non load-bearing partition walls
- wet-applied wall plaster
- external render and external vertical tile hanging."
The render defect is more likely to be an issue with the workmanship during application.
The problem is that the full costs must be higher than £1000. If it is less and the NHBC reject your claim. In the end it may be cheaper to get the render fixed yourself.
I suggest you use Webber Monocouche next time.