I have seen many developers spend weeks building a SaaS that never gets users. Before you start coding, how do you validate whether an idea has real demand?
It's really simple. If the software or app you're working on already exists and is successful, you're wasting everyone's time designing your own. Even if the demand is there, research whether what you're about to work on already exists. And no, in this day and age, copying someone's idea and making it better doesn't cut it.
You write a business plan just like you do for anything else. You look at the competition, the barriers to entry for future competition, can you sell it on your own or do you need a reputable name attached? The more time and more risk that's involved then the more detailed the business plan needs to be.
Create a quick landing page with a waitlist first. Run $50–$100 worth of targeted ads or share it in relevant communities. If people aren't willing to drop their email or pre-order before a single line of code is written, that’s usually your answer.
I try to validate the problem before validating the product. First, I talk to people in the target market and look for a problem they are already spending time or money trying to solve. Then I test the idea with something lightweight, such as a landing page, prototype, manual service, or even a pre-sale.