Yeah, it's up to the vendor to provide and if required, any for any such certifications that the property and any alterations are compliant.
Banks in particular seem to be getting a lot more strict on obtaining certs for any work carried out prior to releasing funds. You may need to engage an engineer to offer an opinion on compliance, as it will be impossible after the fact to get full certification on the steel work that is now enclosed unless you can trace the original engineer (if there was even one involved) and get them to provide the cert which is very unlikely.
A purchaser will often engage a surveyor or engineer to perform a pre-purchase visual inspection of the house at their own expense.