You say there is an extra square footage to the original house?

It involves using sites such as mapliv.com that aggregate all the other rental listing sites and finding comparable properties in # of bedrooms, square footage, location, and quality (granite countertops, stainless steel appliances, etc.)

Be aware that the net clear opening of the window (in width, height and overall square footage) dictates if the window is egress compliant.  

You can do choose to calculate by square footage of unit or number of residents per unit.

My newer home kept jumping around because comps in the area with similar or smaller square footage but lakefront were selling for $100,000 more.  

@Tyler HillZillow's pricing algorithm is a big trade secret but there's a few things you can tell from the outside that can cause differences:1) A low previous market sale on the subject property2) Age of the property3) Inconsistencies in the tax records such as improperly reported square footage, bed/bath count, and lack of garage, etc4) Property owners can update Zillow manually to reflect things such as a big remodel or incorrect tax records

Some have converted hotels to micro units which have very high square footage rent rates.Denver has opportunity and you have to pick and choose.

. - less new square footage per additional unit)?

I didn’t think about the square footage breakdown and how that would be much expensive.

Each condo unit's square footage is used to determine the percentage of ownership each unit owner has in the association.