Might be that the 2nd floor is keeping your exterior walls in place. Consult a structural engineer.

If you want such a high ceiling in a one-story house there needs to be balloon framing to the roof, so the advice to talk to a structural engineer is good. You could end up having to re-frame the entire house!

I appreciate the comments. I will certainly meet with an engineer if I move forward with this, but just wanted to get some feedback on the concept at this early stage.

Unless you actually own the house, as opposed to the bank owning it and you paying the mortgage, the idea is dead in the water. A teardown would be cheaper. You could at least attempt to give lip service to some type of HVAC control then. Really, it would need to be approached more like a commercial build than residential. With the commercial building costs.

I own the house. No mortgage, but based on the little response so far, looks like it's just too pricey an idea for the result. With regard to resale, yes, it would be crazy in a normal location, but the house is on a mountaintop in Malibu. It has a spectacuar view and buyers (myself included) are fairly eccentric. Number of rooms doesn't necessarily matter to a young actor, writer, producer, etc. For them it's all about the WOW factor. In any case, Im greateful for the thoughtful comments.

Has your idea materialized? I am planning exactly the same crime

I am planning the same thing, too.

I have a stick built house too. I want the whole top floor of and build an addition to the side of my house. As I'm now a wheelchair user I cannot get upstairs. So it is impurities for me. Or home is owned free and clear so to fund it we will remortgage.

I should not think it will be that expensive.

But I will of course get an engineer to assess the possibilities

I also am looking for a property with over 15 ft or higher ceiing that I can renovate with a classic urban loft with mezzanine for bedroom. Or, I would have to do same thng as discussed above, remove the second floor which almost always gets into structural inegrity issues and expense goes up, Frustrating. I want to be inside an apartment building, not free standing home.

No, I di