It will come up in a future search for open permits on the house....and then you will lose square footage value for one.

For square footage I try and stay between +/- 250sqft.

Zestimate just looks at the cold, hard facts......square footage, price per square foot, etc. 

For example, if you are building an ADU, you want to make sure there is enough square footage in the lot, so you can filter by x sqft and above on the software.

A JADU can be treated as just another wing of the house and included in square footage, etc.2) Keep renting!

However, they do mention, ad-nauseum, that you if you get a little creative, like adding a room to existing square footage, MTR-ing or STR-ing the property...then yes, you could find cash flowing properties.

Their software does NOT look at your individual house but they price it by the subdivision/street and adjust for various items, like square footage, permitted improvements ect.  

Make sure that it either was added with permit or is not counted in the footage.- hardscape is expensive.  

Another property just sold above asking price in my neighborhood (with less square footage and only single car garage as opposed to my 2 car) for 25% more than what I paid 1 year ago. 

I would look into these guys https://www.submetersolutions.com/If that doesn't work, just split the usage based off square footage and charge the tenants in proportion to that - not 100%  accurate but good enough.