A real rough and ugly way to see what it will add is look at the average sq ft price for you neighborhood and then multiply that by the space you are adding to your home.

Yeah, I actually gave them a break-down of what I was expecting based on a 10% API where I divided by 12, and then multiplied by 3. 

I am on a hunt to find the best SFR's and Multi rental deals based on the following: Cash on Cash Return, Gross Rents Multiplier, Gross Yield, Cap Rate, Debt Coverage Service Ratio, and Internal Rate of Return (10 year). 

At 275 your gross rent multiplier would be about 88 which is really good.  

*Income-Expense Ratio = 1.13%, Gross Rent Multiplier 6.94%*Debt Coverage Ratio = 1.17%Seems not bad -- the only disturbing thing is the cash needed is $61K.Any additional thoughts?

Quick and dirty... take gross rents per month, multiply times 12 months of a year, to get yearly gross income.Then subtract 50% for expenses, or if you want to work and manage the property yourself, including light maintainance(for free), subtract 40%.

Part of due diligence is to call the county and ask for current taxes, and tax multiplier or how the taxes are being calculated. 

You can get a good idea of the cost per sq. ft. and multiply that average number to the square footage of the property you're interested in.For example, if the triplex is 3500 sq. ft. and the average sq.ft. number for similar properties you've come up with is $100: 3500 X $100=$350,000.Of course, there are other factors such as lot size, year of build, etc.

Now multiply the number if piers by $250. 29x250 = $7,250.Now add $200 for permit & $400 for Engineer's letter (yes, both of these are required by the City). 200+400 = $600Total foundation price = $7,850.

And it will continue to grow n multiply until I have a loss.