What 'fail' actually means
Building Control rarely issues a single dramatic refusal. Instead they list non-conformities — items that do not meet the Regulations and must be put right before a Completion Certificate will be issued. The list is usually written, dated, and references the specific Approved Document being missed.
Typical extension failures
Insufficient insulation, missing or undersized lintels, drainage falls outside tolerance, build-overs without a public sewer agreement, and missing fire-stopping where compartmentation matters. None of these are unusual. All of them are recoverable. Most of them are expensive once the work is covered up.
Putting it right
Most issues require opening up work that is already finished — lifting floor finishes to check insulation, removing plasterboard to verify a lintel, or excavating to confirm a drain run. The cost is borne by you unless your contract pushes it back to the builder. Get the failure list in writing first, then take advice before signing anything.
How to avoid it
Stage-by-stage independent oversight catches these issues before they are covered up. A second pair of eyes on insulation before the plasterboard goes on costs a fraction of stripping it back out.