You need a darn good architect. That’s a Frank Lloyd Wright style home with strong architecture. Any changes need to be sensitive to the original design or it becomes a remuddle.

You can't crowdsource this. As Hallett said, you need a very good architect. We can comment on garden variety additions to basic tract homes, but your house is going to require a lot of thought to get right. I put an addition on a 1945 home (my previous house) - on the back, so the front facade didn't change. I engaged an architect and what she came up with was 100% better than anything I could have conceived and I have been drawing house plans since I was a child (as a hobby). We solved a half dozen issues with the house with an addition that only increased the square footage by 10%, and it was more attractive on the back when we were done than we started. You have an opportunity to either increase the value of your home, or decrease it with a poorly thought out revision. You pay a professional to increase it.

Have you checked with your town/city/district to find out what you can and cannot do in terms of height restrictions and additional square footage.

To start you will need an architect, structural engineer and an interior designer. (do not mistake an architect for an interior designer);

Hopefully you have already done a lot of research for what is a very big and probably expensive project.

Wonderful house. To add to excellent comments above, you'll need to move your chimney or rebuild it if you want to build on what's now the rooftop. When you do that, new building and safety codes for fireplaces will kick in if your area is like my area.

Your foundation and walls may need to be strengthened to hold the additional weight.

Ditto, you need an architect. That said, I'd be tempted to repeat the facade on the second floor, except scaled down and set back.

Not the house we all think it is. Only the windows and overhang reference Wright. The rest of it , well, not so much.

Your second floor might look similar to this, provided the structure can support a second floor.

That fireplace is something else!

Thank you for responding, everyone! An architect and engineer are enlisted already. I was just hoping to get some external opinions as well.

I did intuitively understand the need for sensitivity in the design and your responses help re-enforce the importance.

Yes it is Frank Lloyd Wright inspired. In fact it replicates in many ways the Avery Coonley Playhouse pictured below. This is a unique design as far as I can tell.

It would be great if someone could point me in the direction of another project with similar characteristics where a second story was included or added later?