You would get most of the cost out of design by automating the engineering. Whether the customer prefers a pure AI design or spending .001% of the bulding budget on an architect will be a matter of taste I guess.

I think the stakes are higher for a building than most slop. Someone still has to pay for it.

In fact, a lot of stuff is already slop, largely because of the design and engineering expertise required. I don't think one-shotting "design me a house in the style of Frank Lloyd Wright with these specifications" is that far off.