Overall I wouldn't put too much weight on square footage alone. I have been in 2000sf houses that feel bigger than 2500sf houses because it was better designed and had better flow.
As far as valuation goes...square footage is very limited. Quality of finishing, features, location, etc will all play into the value.
As other agents have said, there are many different things to consider (condition of home, improvements, location, etc.). Think about it. A 3,000 SF home in a gated community, built in 2010 with granite counters, stainless appliances, pool in the back yard etc is going to cost a lot more than a 3,000 sf home built in 1950s, no updates, in "bad" neighborhood.
If you haven't done so already, have a Realtor represent you as a buyer's agent. He/she will be able to pull sold comps for the houses you're interested in, and can give you an idea of what Fair Market Value is.
best of luck
The reason we see $/sf on reports is a vestige of the early MLS systems, whose programmers took delight in being able to create reports with these calculations. It has nothing to do with the standards of residential real estate evaluation.
All the best,
My experience is that the square footage that you would calculate would be the entire 3000. That being said, I have not been seeing value based on price per square foot in my area. The biggest factor in value is the location and condition of the house, not the square footage.
If you have a 3000 square foot home in a 2000 square foot area it would be very different than if you had a 3000 square foot home in a 4000 square foot area.
Two 3000 square foot homes could have different values depending on it being a fixer/foreclosure or a home that was either new or had been totally upgraded.
Many factors affect the value.
The house is worth what someone is willing to pay. Supply and demand, interest rates and financing. Many factors including square footage.
My approach would be the following: find out what homes in the 3000 sf range are selling for recently, in that neighborhood, and with similar features. Use these comparable properties to derive a value for the property you are interested in. A qualified buyer's agent should be able to help you with that.
Then, based off of this derived value, deduct what you believe to be the monetary value of having a poor floorplan and sloping ceilings in the 2nd floor. This should give you a ballpark price for what you would be willing to pay for the home.
if you like to get a price per sq/footage just go to some sites which lists sold homes like tulia or redfin and look at the house you are interrested in and those sold around it to get an average price.