Please please please, set your aim really high: think International. Look into project management and scheduling. Look beyond the web app, and certainly beyond the typical $5/mo "monetization". Don't hesitate to offer customized versions and certainly solicit and implement user feature requests. Look into all the industry standards, laws and regulations. The more you learn about construction the better it will reflect in the quality of your product. Reach out to industry groups that your users are members of. Read their trade publications. Attend their conferences. Befriend architects, and actually become friends with them.

That's what I want to tell my younger self when I was working on my first start up, 1999 - 2001.