I'm not making light of your situation, but that of the appraiser and the bank.

I think that this would fall under the category of "functional obsolescence" and I'm pretty sure that @Allan Glass could probably shed some light on it for you.

Show off that beautiful tile work you spent so much money on - plus with that block window, the light will fill the bathroom so much better, where a shower curtain would block that light more.

You got in the deal light (with only $3,000), with seller financing (subject to + 2nd mortgage) creating a nice monthly cashflow ($4800/yr on a $3000 investment is 160% cash on cash return!).

I'd think a place like ABC or another supplier could shed some light on the product cost if that's what you're looking for.I've looked at places with flat roofs, however have always been fairly leery...are you just having normal maintenance performed, replacement, repairs? 

One other thing you may want to consider is to switch all appliances to energy efficient models, switch light bulbs to CFL or eco-incandescent bulbs. 

Thanks a lot @David Light :)

After driving through the back alley, the back unit has been tagged and needs a paint job, as well as security lighting and maybe a spray-paint-resistant coating?

You will quickly learn to do all the little things (changing light bulbs, tightening handles, putting up blinds).

@David Light- thanks for the suggestion.