Your concern is justified. Ask the title insurance company if they will issue affirmative insurance covering the two back units as separate rental units. They will probably say no but there is no harm asking. You will need to talk to the codes officer to determine if proper permits for the units exist. Get the code officers report in writing.
The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change. Attorney is licensed to practice law only in the Commonwealth of Pennsylvania. Responses are based solely on Pennsylvania law unless stated otherwise.
Another place to check on the status of the back properties is at http://cris.lacity.org/cris/informationcenter/code/index.htm.
This may just disclose permits and status of the "main" house, but it may confirm that the other properties were built without permits.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.