Purchase price: 300,000Down payment 20% : $60,000Mortgage amount : $240,000 (30 years 3.4% interest) Property taxes $190 monthly Insurance : $50 monthly Repairs and maintenance: $54 monthly Capex: $54 monthly Vacancy : 10% (just to be safe) HOA: $12 monthly Expenses total with Mortage should be $1,603 If I can rent this out for $1800 monthly that leaves almost $200 monthly positive cash flow.

Why is there a HOA, and what does it cover?

PITI and HOA payments would work out to $1,052 combined.

@Kyle DaubenmeyerI don’t know which condos your referencing, but, typically there are very high HOA and country club mandatory fees.

Not because of the neighborhood, but the HOA fees and assessments etc. 

Provided that the County/City/HOA would approve, would it be a good idea to build an attached ADU for renting out?

He has not paid.We have told the owner to pay the HOA for the electricity used thus far and arrange for regular such payments in future.

Condos costing $290k with $7500 taxes and $315 hoa rent for $1900 which is breakeven with a homeowner loan.

Is there an HOA?

If your whole model depends on the doing Airbnb, then it's always safer to buy a SFH without an HOA.