A Completion Certificate should be with the conveyancing documents as Building Control approval would have been required. Locate the builders who did the work and ask for their calculations. Also, ask if Building Control were notified and whether they were issued with a Completion Certificate. If a Completion Certificate was not issued, you can apply retrospectively. This is termed ‘Regularisation’ and it allows Building Control to check the completed work.
As the works appear to be totally internal, planning permission would not have been required.