You can value these homes on a blue-book or GRM (gross rent multiplier) but definitely do not cap their income. 

File the protest and get a phone consult with themBell county sometimes wants to use a rent multiplier to determine value, so do not give them the $$ that you rent the property for as that almost never works in your favor

But as babies start to grow and multiply, that nice family has to seek something larger.I also anticipate that I'm going to be marketing that 3rd bedroom on my current rehab as a home office now that many more people are working from home.as to your heuristics, pick your areapick a sq ft range eg 1000-1300 sq ftget an agent to compile a couple years' worth of sold data - in spreadsheet form that you can sort.depending on your local MLS rules, you may be able to do this yourself on Redfin.with a little practice, you'll be able to find the properties that sold " as is" then went back on market after flip.look for the ones that added bedrooms or bathrooms to the existing square footage.then look at those that flipped without changing the room count.

I will then multiply that average by the price/sq ft in the subject property.

You could also go the passive route and build a syndication ladder, where you hope to pull your initial investment out every 3-5 years and multiply that into other syndications (not always possible).

Would I just simply multiply all three properties and then subtract with the new refinanced loans ($450k - 337.5k = 112.5k equity)?

Multiply that increase by the 150-300 plus units and that facility and it really starts to get exciting! 

., Is this simply the average cost per square foot to build in the specific area I am looking to invest multiplied by the total square feet of the multifamily building?

You may be thinking you know how much a tile install costs because you looked at Home Depot and tile is $4.99per square foot...so, simple multiply that by square footage.

That's the wealth you can gain by multiplying your property and using leverage.2.