If the protection you think is in there is in the deed restrictions for the neighborhood, you can do three things.
1) Contact the Home Owners' Assoc. and have them enforce the restriction.
2) Contact the City Neighborhood protection
3) If you live in the neighborhood, you can file suit against the home owner.
Many neighborhoods contain single family housing, some of which have been rented after the owners moved out.
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I would first make 100% sure this is not a violation. Sometimes renting a garage apartment or an extra bedroom in a single family home neighborhood is allowed. There is wording in our standard listing agreements that Realtor can not violate area rules/regulations they are aware of. Examples including restrictions on signage or times to show a home. Of course we cannot discriminate.
The owner would be equally responsible if there is a violation. Is this a deed restricted community? Inside a city jurisdiction? You can report what you know to them and see what they can do, but you likely will not get much help as they have bigger problems to deal with.
As we always mention here, when seeking legal advise, speak to a real estate attorney.
Bernstein Realty, Inc.
Sounds like this could be perceived as a "zoning" violation. Your concern should be directed to your local municipality for their response. It's also quite possible that they could be doing nothing wrong. Get the facts before offering the punishment.
I'd check with planning and zoning in your city or your city council person to see if they see it as a problem or something they want to tackle. The owner of the home may have committed a violation of the P&Z ordinance if there is one....for example if a single family residence has been turned into a duplex, or the owner has created a garage apartment without a permit....and then rented that space. It's the owner you want to go after, not the realtor. The lease would be between the owner and tenant and likely the realtor is not even a party to the contract.