One night during autumn when the A/C is not needed (and is disconnected from the electric panel to reduce utility bills), I hear a noise of a machine or some engine running all night long, very similar to A/C.

The roof estimated repair cost is $32,000, I estimate the septic  is $30,000 since it failed completely, and other repairs (windows, doors, paint, floor refinishing, furnace, electrical) to be about $30,000.

I use sub meters for my commercial property - for electrical.

They pay electric.

Separate water and electric for each unit, all underground plumbing replaced in last 10 years. 2 sewer taps, 4 units on each- this is the only thing I could see giving me an issue of blame game if something happens. 7 units occupied with long term tenants, 1 unit needs replaced which we will replace with newer home and sell to new owner/tenant.

Both Units - New roof, electric panels, windows, lava rock landscaping and small patio in fenced in back yard.

The cost will go up as square footage increases or multiple outbuildings etc.Big things to look for in an inspection: the foundation, the roof, the electrical and the plumbing.

If you pay for electricity in the units, its a direct benefit to you. 

@Geneva Crisostomo It's best to make sure the infrastructure is in place before advertising (i.e. lot paved and/or concreted, access to water/sewer, electric, gas, etc).

In either scenario, double check the cost of taxes, water, sewer, electricity, gas and insurance.