Rents are at record highs across the country. But even if apartment landlords do manage to fill up most units most of the time, finding other streams of property income will only increase cash flow from month to month.

Consider some of these options that might help boost your bottom line.

Apartment buildings.

Image Source: Getty Images.

1. Offer additional storage

Storage units can cost anywhere between $100 and $300 per month. Whether you build your own storage space on-site or you strike a deal with a nearby self-storage company, offering secure storage space to your tenants for an additional fee can help generate extra revenue.

2. Permit pets

If you previously didn't allow pets at your property, permitting them now would likely increase your renter pool, as many people started or added to their furry families during the pandemic. Charge a pet fee so you can amass the funds needed to clean and make any repairs to the unit after the tenants (and their pets) vacate. Unlike security deposits, pet fees are nonrefundable -- and pet owners are more than willing to pay them for their furry roommates.

Already have a pet policy in place? Consider adding a menu of services catered toward pets and their owners, including dog walking, pet sitting, or pet grooming services.

3. Lease additional parking spots

If you've got a large-enough property and already allocate a spot per unit, offer lease options for a second spot for an additional fee. If there's a public transportation station nearby, consider renting spaces or placing meters in a section of your lot to allow commuters to pay to park during the day.

4. Add a billboard and collect ad revenue

Zoning laws -- along with a bit of common courtesy for your neighbors -- will determine whether you can add advertising signage to the roof or exterior of your building. If you've got a multilevel apartment building, your roof could be a new source of passive income. If you haven't already been contacted by an advertising firm, reach out to one yourself to learn about installation and revenue potential.

5. Install vending machines

If you've got the common space for them, vending machines have long been a convenient way to grab a beverage or a snack. Adding a refurbished one to your lobby, laundry room, or another common area will cost around $2,000 if you buy it outright. You could also rent one from a vending company, which will take a cut of the sales. Your tenants will appreciate having easy access to cold beverages and tasty snacks -- bonus if you install a machine that allows for credit card payment rather than just taking coins or bills.

6. Lease washer/dryers for in-unit laundry

Coin- or card-operated laundry has always been a reliable source of income for apartment building landlords. No resident will balk at having an additional washer or dryer added to the laundry room, so consider adding or upgrading appliances at your property. 

But if there's a washer/dryer in each unit, you could rent the appliances to the tenants for an additional charge each month -- which many likely will be glad to pay. According to HomeAdvisor (owned by ANGI), it costs between $350 and $600 to install a washer/dryer hookup. Depending on the type of appliance you get, it's likely the installation will pay for itself within a year.

7. Rent a spare unit as a work from home space 

Short-term rentals are a good way to fill in between long-term leases, but it's understandable that you might not want the hassle of people moving in and out so often. Instead, consider renting out vacant units as remote office space. Someone in your building right now might jump at the chance to rent out, say, a studio apartment to use as office space. With minimal furniture and a working kitchen, they're in business.

The larger your property, the more opportunities you have for alternative streams of income as a real estate investor. But even landlords of smaller buildings stand to make more money each month by finding additional ways to add value to the tenant experience.