I am going to get clobbered for answering a question way out of my comfort zone - Marin, California (How far can I be). But I got curious after I read your other question about full service broker vs. Flat fee.
First, your house sounds pretty cool to me, mid century modern, after I just volunteered as a docent in an Eichler home here. It seems to me that you have also thought about a few things, marketing a 2 beds vs 3 beds and eventually whether the appraisal will hold or not.
Personally, I think the idea of having the third bedroom definitely helps you market your house. However, I wonder where the third bedroom will be and if it will fit in well with the structure and the flow of your house? I think an awkward addition maybe more hindrance to the sale of the home, so you might want to think about that, before adding on.
I also wonder if you are going to add the third bedroom, why would you not update your kitchen? The Eichler home I was in has a gorgeous remodeled kitchen (lots of ooohs and aaahs). He had this zebra pattern cabinet from Home Depot Designer Center and brushed steel patterned counter and some cool lightning, really adds to that modern feeling. Kitchen upgrades definitely helps sell a home.
How about the bathrooms, are they updated? Staging to showcase your modern home and outdoor environment? Your in ground pool and Spa, are they in a great condition which add to the enjoyment when you look outside, or do they need to be serviced?
All these are some questions that are very difficult to answer on paper in a distance.
So, my question is, with so much in stake â€“ the cost for upgrade, the sale of your home - why don't you have a Realtor to come over and give you some suggestion? Although it does sound to me that you are pretty savvy in this, it also does not hurt to have somebody take another look and discuss with you in person.
We mostly do this as an added service to our potential clients. Usually there is no obligation to use the specific realtor, but if you like his/her service, then you might want to stay with her/him.