(The arrangement was that they pay me for electricity because they share electric with other unit.) and they started to complain about roof leak which caused them hardship.

Seems renovating this part of the attic would require ceiling insulation, drywall, flooring, electrical outlets, & heating source.My concern is that a reappraisal wouldn't add anything to legal square footage. 

You'll either have to convert the house into a legal duplex by getting permits from the city or you install an electrical panel split so that you can track each units/bedrooms of their power usage.In terms of lending world and being to use the rental income, the property needs to be a legal duplex meaning title must be shown as a multifamily and not a single family in order to use the rental income separately.

If it was built before, say, 1960 there may be a slew of electrical work you'll need to do to stay in regulations with current times and, of course, for the safety of a future buyer.

Before I bought the house, I found a 3 inch pipe with a spicket and electrical wiring going to it.

The foundational items, roofing, HVAC, plumbing and electrical were newer and needed very minimal replacing and updating.

I am very pro EV and electric vehicles, we want the tenants with these cars. 

Ceiling fans are good on days where you can get away without using an AC (i.e. saving electric consumption).

It is located in the fort myers area.3 bedrooms2 bathroomsRecently remodeled, new A/C, new floor, new electrical, new bathrooms and kitchen.

I could theoretically run my own electrical as well, but I would never do that because there is nothing DIY for that, and there would be a serious risk to doing that - so I had a certified electrician run my electric lines.