Rental property newbie: What hidden costs did you discover?

You'll also need insurance both to protect the property (not the renters possessions, I suggest requiring them to have renters insurance for that) and liability coverage.

Make sure you know which things your HOA/Condo fee covers and which it doesn't. Also investigate the financial health of your HOA/Condo association since insufficient reserves can result in special assessments (they collect a pile of money from you when they come up short for major repairs).

Be financially prepared to weather periods of vacancy or non-payment/eviction.

You'll want to budget for repairs and maintenance (carpet gets ruined, appliances die, roofs fly off, pipes burst, etc). You could review age and average lifespans of each major item and calculate an annual budget for their replacement if you wanted (furnace 20 years, water heater 12 years, carpet 5 years, etc). I found that a bit tedious and unhelpful since breakdowns don't happen on schedule. Instead, I prefer to just make sure I have access to a large credit card limit and reasonably liquid assets to cover whatever might crop up.

If you don't have say $10,000-20,000 that you can access in short order, you might want to make sure you have an easy way to access/borrow against the home's equity.

Research screening methods to reduce your chances of having a terrible tenant, lots of work up front can prevent significant costs.