Property in or around City Heights

[Johnathan J Goodwin]

I found a property that is partially in an Open Residential Zone (OR-1-1) and I was hoping to an add an ADU to this property. Was thinking due to size of the house could easily be 900sqft 2BR detached, but I have no idea what the OR-1-1 will deter. And the land has a pretty good grade so I'm not sure if it will be allowed on that account as well.

Not to mention the current owners seemed to have added a room to the basement with a bathroom. I believe this is not a permitted room as the realtor kindly mentioned as benefit, "it isn't included in the square footage." I don't know until I speak with someone, but I'm curious to if this would prevent the ability to add a detached unit. 

Any advice/contacts are greatly appreciated!