Is there a home plans program that allows you to do house plans? Relatively inexpensive, preferably <$500
Trying to decide where to add onto our house and my husband needs something more concrete than ideas.

It's doable with sketchup, and the good news is there are quite a few youtube videos out there on how to use sketchup. I've also heard, but not tried, that Autodesk (maker of AutoCAD, Inventor, 3DS Max, etc.) has some free CAD software that can be used. Been meaning to look into that more, just haven't yet because I usually use my work laptop, which has CAD and all sorts of fun toys on it.

I did all of our mockup work with Sketchup. There's a bit of a learning curve but once you get going it's pretty straightforward.

Thank you, thank you!

We gained two grandchildren this summer and when they are both here our once spacious LR shrinks quickly. We have a formal dining room that I think we turn into a bedroom. Between it and our garage is a rock garden with a sand base that would be easy to use to build in a bathroom. That would connect to our garage and turning that into a play room/dining area. My husband wants to build a steel building/man cave to become the garage/storage.

Sketch up is cool and i recommend its use for lots of stuff but simple.architectural cad package like chief architect would be far easier for this. It also understands building code issues and will build materials list that sketchup does not.
Real architect will laugh at chief architect but it will quickly get you from idea to view and the entry $100 package is fine for what you described.
We have proe solid works Autocad and all the open and low cost cad systems available here. But when my wife wanted to layout the barndominium CA was trivial and pointed out lots of issues that sketchup and even pro 3d cad packages like solid works and pro e would not.

Our company has used Sketchup and HomeDesigner2017. They are both great tools for drawing up our renovation plans and showing them to customers.