ultimately evo and id are both not falsifiable, that's why we test which models better..
when you say only knowledge of design is evidence of design i could say only observation of evolution is its evidence..
a house or more could always (vanishingly) potentially arise spontaneously undesigned. not the proof but the preponderance is the question..
since a house contains much order (mathematical macrostructure, multicomponent functions like hvac, etc) we conclude that design is likely wo fearing disagreement..
but evo can't do the same for biology!!
Actually, evolution is falsifiable. For instance, if paleontologists uncovered a mammal-bird transitional, evolution would go right out the window. ID, on the other hand, is not falsifiable because everything can simply be explained as "that's the way the designer did it."
First of all, evolution is not equivalent to saying that there is no design. Evolution is simply the theory that the variation over time of allele frequencies in populations of organisms is sufficient to give rise to all the diversity of modern life. It says nothing about design or lack thereof, because, as I stated earlier, questions of design are not falsifiable and hence not scientific. Please don't confuse the science with the scientists, many of whom I will admit push an agenda that seeks to eliminate the possibility of design. Many scientists and others do indeed use evolution to argue that design is not possible WRT life, but that's not what the theory of evolution actually says.
If you restrict the question to just what the theory of evolution actually says, then it is indeed falsifiable and hence is a scientific theory. There are all kinds of observations that would either completely falsify evolution or lead to major modifications in the theory. Just a few examples: find a new species of life which doesn't utilize polynucleotides as its genetic material, find a fossil of a modern mammal that can be reliably dated to an age of 1 billion years or find a fossil that is a transitional species having characteristics of birds and mammals (both of which all other evidence indicates are branched off from reptiles.)