First, Rentometer is great for a ball-park figure, but it doesn't take into account a lot of information on the house (type [SF vs. duplex], square footage, condition, finishings, etc.)I would recommend reviewing rental listings on Zillow/Craigslist that are comparable to your in-laws house so you can get a better idea of what rents would be. 

Total square footage of the 4plex building is 3,376 sq.ft. 

Sq footage of restaurant is 1623. 

@Brian Stennett Sr adding square footage, adding a second bath, kitchen reno, bath reno, fresh paint.

It is hard for me to give much advice without knowing the property address, number of bedrooms and bathrooms and the approximate square footage.

You have mentioned a lot of fun facts, but nothing that I would use to analyze a market for REI.In all these areas, what are numbers ($$$$) for the following important questions:1 - What are the entry prices for the common square footages (ranges of similar comps)?

I'm hoping that some "big pocketed" Angelenos on this forum might know the answer ;)As an aside, while I'm of course grandfathered in the sense of not having to tear down my house, I'm relatively sure that you can not add any square footage or do major remodeling without adding a second spot, i.e. complying with current parking regulations.

Often, floor plans need tweaking, square footage needs to be added, and even that rooftop deck with a view can do wonders. 

After looking back, my regret is not taking advantage of the maximum allowed sq footage of 800 sq feet. 

If ONE unit is vacant and both units are equal in rooms/bathroom composition, then the new owner will need to move into the vacant unit and then the owner can either evict to take the occupied property completely off the rental market (5 years off the rental market), evict to take back the occupied unit for personal use like moving in immediate family (2  years off the rental market), or they can evict given the tenant is a nuisance or conducting illegal activity (this is a harder method to evict since you have to provide evidence and have a case but if they're selling drugs and you have video footage or something, then make your moves) and there are a few other reasons for at-fault evictions.