It is remarkably difficult to find anything resembling a derivation of this term. The closest I got, both in the OED and in the copious slang dictionaries available, was that "as safe as houses" is a British term meaning "perfectly safe". The term apparently arose in the mid-19th century, but other than that, my only direct resource is "origins unclear". So! Example and conjecture it is, then.
My initial understanding of the term is that it refers to financial security: land was the source and evidence of personal wealth throughout western civilization for a good long time. A house is a serious investment, but it is a very safe one, especially as compared to such fantastic economic blunders as the South Sea Bubble. A house is not going to inspire a flurry of financial speculation; a house is not going to have shipping problems; a house does not have to return x amount for the stockholders to be satisfied.
A house does none of these fancy financial things: instead, it provides the owner with a place to live. Property is an investment, but one without many of the speculative risks typical of investments. A house is a simple, solid thing. As long as it doesn't burn down (and you don't mortgage it to death and get foreclosed on), there is no way you are going to lose it without some recompense; even if it does burn down, you still own the land on which it stood. Even if you lost all the rest of your money, you would have a place to live: a substantial asset for potential sale or mortgage.
In addition, a house is one of the few investments that almost always increases in value over time. This depends on the house itself, the area in which it is located, and the amount of upkeep you do, but, in general, houses appreciate. They will always give you some return.
So. A house is safe because: 1. you are not going to lose it on speculation and 2. you are almost certainly going to realize some return on your investment. So far, so good. Let's take the OED's examples for this phrase, and see what kind of evidence we have for my conjectures.
Under "houses":
- 1871 Hardy Desp. Rem. III. iv. 92, I shall be high-treasoned--as safe as houses.
- 1932 A. Bridge Peking Picnic xix. 238 Kidnapping Frenchmen is simply too unremunerative for words, whereas we're a perfect gold mine, safe as houses.
I interpret the first sentence to mean that the speaker is entirely safe because it would be high treason--figuratively, I assume, but it doesn't really matter here--to harm him/her, and no one wants to come up against a high treason charge. This supports the certainty of safety, but it doesn't have much to do with the financial end.
However, the second sentence seems to support my hypothesis. It clearly refers to making money, and the surety of such income on two occasions. Where ransom is not certain, and potential failure involves various punitive business, the second, unstated option offers a certain financial return.
Under "safe":
- b. to be safe, followed by inf. or const. for, is predicated of a person or thing to express the certainty of the fact or event involved in the predication.
- 1894 J. S. Winter Red Coats 50 You know the Colonel is as safe as houses to come round after church parade.
This construction again supports the absolute certainty that event x--in this case, the Colonel's visit--will happen. There is still no financial evidence, only surety.
So. I still cannot make a strong case for my interpretation, but I can't dismiss it entirely. Although the evidence is sketchy, it seems to me that this is still the most likely derivation of the phrase.