I think that is too strict a view - design can be a shared activity. If the house is built then you and I together share in that design. Depending on the relationship between us even the details of the house may be more attributable as my design. For example, I may have instructed you that it should be a four bedroom, two bath duplex with an attached garage. But at a minumum, it was entirely my design that a house be built on that plot.
I also think you're the one picking nits and for our purposes the "designer" can be treated as a single agent. Let's not wander from the original question: on what basis should we call the result of an evolutionary process a design? As before, I maintain that it can be a design if the result was intended.
Very, very few people would see it that way. If I actually draw up the plans, and you have no input other than agreeing with the design that I come up with, then no one I know would say that you had any part in the design of the house merely because it was you who gave the order to have it built.
And that answers the question about evolution. You might know beforehand that an evolutionary process might result in a certain form, just as you knew before hand that giving the order to build a house would result in a particular form of house. But in neither instance are you the designer, by any common usage of the term. In the first instance the design came about naturally (as opposed to artificially), in the second, someone else came up with the design.
Now, stripped of all the excess semantic baggage, what Darwinian theory posits is that life forms changed into new forms through a process of gradual variation regulated by natural selection. What ID posits is that intelligent intervention was involved at various points throughout (or all throughout) the process of speciation.