If it meets code but simply wasn't permitted, you can probably rectify it with a few hundred bucks to your local government. 

Zip code?

These are professionals with additional training and a stricter code of ethics.

I researched in my area under the muni-codes but the descriptions are very vague.Thanks ahead...

The property is in Los Angeles County, 91390 zip code and I can reply with whatever further details are needed. 

Please contribute more if my info alters your answer.  

Some examples of habitability codes are functional toilets, functional sinks, functional smoke detectors and carbon monoxide alarms, weathertight windows and doors leading outside, bathrooms must have a mechanical vent or an easily opened window, a bedroom must have an easily opened window and either two electrical outlets or one electrical outlet and a light fixture and all pipes, plumbing and electrical must be per code.

Furthermore, I believe there are a handful of tax codes/rules when it comes to renting to family members so make sure you're aware of those.

Quote from @Donald Korman: It depends on state laws and local codes. 

@Rachel H.Yes these are 40 year old MH but originally the county said as long as they were brought up to code we would fall under the grandfather status for shared septic.