I understand it to be a house built with standard specification grade stuff. No custom requests. For example, no ceiling fan boxes, no recessed fixtures, nothing above minimum code, etc. Just the basics. Of course there is speculation whether someone will buy it that way, but I don't think that is the derivation of the term.
Well I can't give you the real answer either, but I thought that "spec" meant that the builder did the specifiying because there was no buyer yet. Short for builder specified. But I never heard speculation as an alternative. hmmmmmmm. I don't know. I still have my chips on specified.
I have done dozens of "spec" homes. They have ranged from plain and simple to extravagant as far as bells and whistles go. A "spec house" is a house built to the GC's specs as a speculation home for the GC to market and sell after it is complete. If you build a dozen tract homes in an addition and they aren't sold to anyone before completion they are spec houses. Conversly, a home constructed for an individual, with options of their choosing, to occupy as their home is a custom home. I have done spec homes that had all the bells and whistles like automated HVAC and lighting, central vac, video, security, heated tile floor, etc. They always sold and I could never figure out why someone would pay $700,000.00 for a new house they didn't choose the floorplan and amenities for. If I had access to that king of money you can bet the house it would buy would suit me to a "T".