How to build your own startup while working full-time at a big company

This can work, but it's better to save your money, quit, and start your company.

Why? Most companies these days (especially Tech companies) make you sign a contract that gives them any ownership of your code that you build outside of work hours.

Even if this isn't the case, you will still need to clear it with the legal department at your company (or risk the consequences later). This can bring other consequences..and limit your future at the company (why would they want to train and promote you..if you are just going to leave).