So what you want to do is get familiar with your local market. by looking at listings online, go to Open Houses, etc. and pay attention to What is the square footage of the house?

I have another site I like so far but can you guys weigh in on the accuracy of https://www.remodelingcalculator.org/     I am also now looking for the specific definition of square footage.

. $650 x 2, and $550 x 1 (10 yr tenant).All units are:  2/1.5/0 with patios, and 2 parking spaces.Ask:  $220k (55k/unit) --- similar aged duplexes are listed for 60-70k/unit, but small MF 8-units in C/C- areas are listed from 28-35k/unit.Total square footage ~4400Taxes:  $2800Owner pays sewer/water:  $250/mo averageInsurance quote I got was:  $3636/yrFiguring 8% for PM, and 10% cap ex and 10% maintenance/repairs.The vacant unit was in very good condition, but had not been updated in several years. 

Unless there is a minimum square footage requirement in the local codes what you are considering may be possible to build to standard codes.

I take into consideration the number of units and square footage as they explained in a blog post Bigger Pockets published (https://www.biggerpockets.com/renewsblog/2015/03/03/why-you-cant-make-money-on-30000-houses/). 

If you occupy 60% of the square footage, then it would be considered owner-occupied commercial real estate.

An appraiser my not include the square footage of the addition in the assessment of value. 

Roughly what square footage of home and price point are you talking about?  

But like I mentioned on the phone, if your plans are short term for the property then adding sq footage and adding the in law without calling it an in law in the SFH to flip would be a good option.

I am a new real estate investor but given the price and sq. footage of the homes being sold, I'd love to explore the viability of the opportunity.