Question: Is there a definitive scalable 3D printer? 

I've seen examples of Chinese companies printing entire houses, and I'm curious as to printers / filaments that are intended (or at least able) be scaled up for (very) large print jobs. 

Since most hobby printers can take hundreds of hours for something that can still be held in our hands, so I'm curious if there are any designs for printers that are meant to extrude material efficiently with a easily scalable printing area. 

Open sourced / free is preferable; though I'm interested in **any** designs that exist, commercial included.

Answer: 
If I understand your question correctly, it sounds like you're looking somewhere within the [RepRap](http://reprap.org/) realm. The RepRap community is mostly responsible for the boom in consumer 3D printing in the past 10 years, and that's most likely because it's **open source**. RepRap designs are mostly dynamic (and most parts can be 3D printed), so you could theoretically build a larger frame for your machine and use a slicing engine that allows you to set the build volume. I believe [Slic3r](http://slic3r.org/) allows you to customize the build space, I'm not sure though.
As far as I know, right now, all the "3D printers" that can print houses, bridges, etc. are experimental models (and sometimes, vey elaborate art projects) - they just don't exist except as one-off creations designed as a proof-of-concept showing this can be done.

Maybe some of those project published their plans and code but they are not designed for mass production or general use, even if you manage to build another copy of one of them it will probably be able to print just the same project the original printed. 

You specifically asked about speed, material and build area:

**Speed:** the speed really depends on the size of the printer's nozzle, the typical desktop printer has a 0.4mm nozzle, if you replace it with a 1mm nozzle the printer will be approximately twice as fast (1.25 nozzle area vs 3.14 nozzle area, assuming you have an hotend designed for larger nozzles that can melt the plastic fast enough).

Now, the "house printer" does not use a standard desktop hotend (see material below) and buildings don't have fine details so you can make the head much larger.

However, as far as I know the early prototypes are still much slower then conventional construction methods.

**Material:** houses are typically not made of plastic, the printers I know of have welding equipment instead of an hotend and extrude steel

There are probably other methods but I bet all of them use typical construction materials such as steel and not thermoplastic.

**Size:** and last but not least, it's obviously not practical to build a printer with as house sized platform, the house printers are actually relatively small robots that travel across the construction project, leaving material behind them and then climb over the previous layer to print the next part.