Web designers always have plenty of projects, especially when you're a professional in your field. Still, additional income is a great bonus to your main work—especially when it’s passive.
SELLING TEMPLATES AND THEMES
One of the most common and profitable ways for designers to earn money today. You can fully unleash your creativity and design something truly unique and appealing to potential buyers. And the best part is, you don’t need to create dozens of different themes—just one good one is enough. Something you yourself would be willing to buy. 
The most in-demand templates are, of course, for WordPress, since it's the most popular CMS platform. Don't forget how convenient such themes are: the client simply finds the template, buys it, and installs it. That way, they avoid the risk of hiring a bad designer who could deliver poor-quality work and still overcharge. So it’s all in your hands!
Still, before starting your own template, check out Template Forest or any other platform that sells WordPress themes. Analyze the top-selling templates, highlight the details that make them unique—maybe your theme will be the next best-seller.
OLD GRAPHIC FILES
Every designer has a folder full of various graphic files and design elements created during past projects but rejected by clients in the end. These files often sit gathering dust among other unused materials.
So why not give them a second life? There are several online marketplaces where you can sell your creations. Just like with templates, many designers already earn well from this. All you need to do is sign up, upload your files, and wait for someone to buy them.
TEACHING
Many people are eager to gain new knowledge and learn web design. Some do it to broaden their horizons, while others want to pursue it as a full-time career. 
For both groups, online learning is often easier than attending classes or courses in person. Help them with that!
All you need to do is create a quality online course or tutorial videos where you share your skills and expertise. The price of your course will depend entirely on your qualifications, level of mastery, and the quality of your content.
That said, don’t forget that the web is full of free learning materials—so your course needs to be better than what’s already available for free. Make it worth paying for, even if the price is small.
BOOKS
Do you enjoy writing and have something to share with others? Maybe you're an expert in a specific area and know more than most? 
Just like with online courses, eBooks are in demand and sell fast because many people find it easier to absorb written content than video. So if you’ve got a knack for writing, try creating a book that’s not only useful but interesting to fellow designers and even your clients. Just make sure it offers valuable, original information—not a rewritten version of something that’s already been covered in dozens of manuals.
CREATING A MOBILE APP
Mobile apps are becoming more and more popular every day, which means designing and launching one can bring solid income. It doesn’t matter whether your app is educational, entertaining, or helps users boost productivity—it will surely find its audience.
Of course, if you don’t have coding skills, you’ll need to team up with a developer, but the result can exceed your expectations—especially if you find a reliable partner.
YOUR OWN BLOG OR WEBSITE
This method is somewhat similar to courses and books. Having your own website, portfolio, or blog is not only a way to showcase your work and skills but also a great way to boost your income. First, you can earn from ad impressions via search engine monetization. Second, you can sell ad space directly to clients or other interested parties. 
The biggest advantage of this passive income method is that your blog can earn money 24/7. The key is to keep it updated and filled with engaging content.
Yes, at first you’ll need to put in real effort to make your blog popular and well-trafficked. But it’s worth it. Plus, during the process of building and growing your blog, you’ll gain valuable skills like web development, SEO, copywriting, and more.
IN CONCLUSION
Don’t assume that as a web designer, you’ll only earn money by building websites. The skills you develop during your training can open many new doors and lead to substantial earnings—if you approach it wisely. Also, during slow periods without client projects, you can still improve your skills by designing and creating your own concepts, which you can later sell on design platforms.
Designers have many ways to earn—and even more when it comes to passive income. It’s all in your hands. Go for it!