I use gross rent multiplier as one of my quick initial metrics, and I aim for a GRM less than 10 typically.

Multiply that with sf of your house.

Nonetheless, you should be able to just search other comps in the area figure out what the $/SF of other houses are, and then multiply that number times the number of square feet you are adding. 

It's a whole different beast.LA is unique in many ways: an extremely constrained inventory, an extremely high gross rent multiplier (GRM) in most submarkets, a rent control ordinance, massive barriers to entry in both cost and permitting.All of these can go under "Pro" and "Con" category depending on how you approach them.

Calculate the monthly additional cash flow you would have, multiply it by 12 to find the annual cash flow3.

I can get the basis value by: "To determine the basis of an individual lot, multiply the total cost of the tract by a fraction.

Also, it should be fairly easy to determine the square footage by measuring the length and width of the ADU and multiplying them. 

Below are the turn costs for each segment based on the above assumptions.If I multiply Total Vacancy Cost by the 10 year turn frequency, I get the following:Transient: 10 turns x $3,200 = $32,000 or $3,200/YrPermanent: 2 turns x $2,000 = $4,000 or $400/YrTransitional: 5 turns x $7,000 = $35,000 or $3,500/YrAs you can see, to have the same net cash flow from the three different segments, you need to have a far greater return from properties that target Transitional and Transient tenant pools than the Permanent tenant pool.

The potential profits for the duplex are pathetic because the re-sale value for the duplex does not revolve around the Gross Multiplier and the re-sale for a 4-plex does.

No, I don't personally use Shiseido because I'm already too wrinkly.Suppose, you own a 4-plex or more units and today for most properties the Gross Multiplier is 18+So, when your tenant moved you just earnedAdditional Gross Income of $1800 per year X Gross Multiplier = $32,400 is the amount you can add to the price for your property if you want to sell it PLUSAt the end of the first year you earned $32,400 + $1800 from additional rent = $34,200Now, lets look at this if you buy 4 units, increase the rents $150 for 4 units the day you buy the property, increase the rents $50 every year and I will assume you are currently cash flowing not less than $200 per unit.You are going to be FILTHY RICH since you have the common sense to increase rents!