The bad news is your kids have finally flown the nest...but the good news is your kids have finally flown the nest! Changes to your home life can be bittersweet but there are plenty of reasons to look forward. To start, you'll suddenly have all kinds of space in your home, places that you can use to earn money and grow your retirement fund.

Here are four ideas to inspire you, each with a different time and financial commitment, for turning your empty nest into a healthy nest egg.

1. Rent out storage space

It may sound too simple to be true, but your extra shelf in the garage, space in the attic, or corner in the basement could be earning you regular income. With living space at a premium, many people need extra space, but traditional storage companies are often inconvenient and expensive.

Enter peer-to-peer storage, a system where regular folks rent storage space from other regular folks (like you!) Companies that automate storage rentals in the United States include Spacer and Neighbor, which both work in ways familiar to anyone who has dealt with AirBnB.

Investment: Limited. Aside from being clean, dry, and secure, the requirements for a storage space are minimal. Listing is free, with a processing fee charged on payout.

Potential earnings: Peer-to-peer storage apps allow you to set your own price but they recommend offering space at around 50% less than traditional storage units. According to Spacer, you could earn between $100 and $150 per month by renting out a shed.

2. Rent out a parking space

Your extra parking space could be a cash cow, especially if you live near a school, arena, or airport where parking is limited. In addition to managing storage space, Spacer joins other apps like Stow It, ParkStash and Pavemint to broker parking bookings.

Investment: None, though homeowner's insurance varies by state so interested renters should speak to their agents about whether they need to buy an extra rider.

Potential earnings: ParkStash claims owners can generate up to $200 per month, which is in line with Spacer's estimate of $2,000 to $4,800 annually for a garage.

3. Rent out a room on AirBnB

AirBnB is the peer-to-peer app that revolutionized the accommodations industry by empowering people to rent out their spare rooms or apartments, usually to short-term visitors. For owners, offering an AirBnB is a relatively easy way to generate cash from a spare room.

Investment: While there is no upfront cost for using the app (AirBnB's service charges apply to renters), be prepared to invest in items that make the room comfortable and secure. These might include a quality mattress and bedding (~$1,000), window coverings (~$500), and a secure door lock (~$50). As this strategy involves renting a room in your home, be prepared to invest time in welcoming guests, offering information for their stay, and cleaning.

Potential earnings: The amount of potential earnings will depend largely on two variables: location and amenities. Private rooms in Manhattan, for example, average at around $150 per night, while the same in El Paso runs for about a third of that price. Add amenities like a patio, laundry access, or a private bathroom to increase your earnings.

4. Add a rental suite

If your home has the layout and capacity for a self-contained rental suite, you could create a steady and permanent extra revenue stream while increasing your home's resale value.

Investment: Your home's current layout, size of the suite, and amenities will all affect your investment and eventual return. Refinishing a basement and turning it into a one-bedroom apartment averages between $6,500 to $18,500, while remodeling takes between $10,000 and $30,000. These kinds of projects usually take a minimum of a month and half (though home improvement projects are famous for running long). Most homeowners don't have this kind of money at their fingertips, but many financial institutions offer loans products such as a home equity line of credit that can help.

Potential earnings: The rent you can charge for your suite will depend on the rental market in your city, but bear in mind that once you've paid back your initial construction costs, all rental income can be directed elsewhere

From passive income opportunities to home construction projects, turning your free space into free money has never been easier.