If the extension was built under Permitted Development, apply for a Lawful Development Certificate (LDC). Otherwise, agree with Building Control on what items need to be upgraded and obtain a builders quote for the work. Apply to Building Control to regularise the works that were built without building regulation approval.
If the current owners are unwilling or unable to do this before you purchase the house, negotiate a reduction on the purchase price and/or ask the sellers for indemnity insurance.
Unless it is a dangerous structure, Building Control would find it very difficult to take enforcement action to regularise the works.