Can I get retrospective building control approval?

Yes, you can get retrospective building control approval. If you didn't apply for building regs approval for the work before, or perhaps building work carried out by the previous owner didn't have the relevant approval before starting, you can apply for 'regularisation' – retrospective approval.

This involves a local council building control surveyor visiting the site and assessing the work to see if it's up to standard and if not, recommend improvements to bring it up to standard so they can issue the appropriate certificates.