In this particular excerpt from "Composed upon Westminster Bridge, September 3, 1802," the speaker personifies the houses of the city just before London wakes to another dawn. They seem asleep along with everything else in the city during the first few moments of light before the bustling metropolis erupts into life.

The houses stand for the city as a whole. As dawn breaks, it is asleep, just like most of London's inhabitants, undisturbed by the ceaseless activity that will soon begin. The city has a "mighty heart" that remains still and calm, as most people have yet to rise from their beds. And the river Thames "glideth at his own sweet will"—note the use of "his" and not "its," another example of personification—as if it were an artery fed by the mighty heart.

The use of an anatomical metaphor adds to the use of personification. The city as a whole is like a human being; it sleeps, it moves, it has a heart. And for now, the speaker detects the heartbeat of the city beneath the slumber of the houses and their inhabitants. Before long, though, the city, along with everything in it, will come to life, and then the houses will no longer seem asleep.

What is meant by "houses seem asleep" in Wordsworth's "Composed upon Westminster Bridge, September 3, 1802"?

In this Petrarchan sonnet, Wordsworth describes the view of London from Westminster Bridge on a quiet morning in 1802 before the usually bustling city is awake and moving. When Wordsworth notes that the houses of the city "seem asleep," he is referring to the fact that they are unmoving, untroubled, and do not seem to look the way they usually look during the day, when the streets are filled with people, and there is business being conducted around the houses.

Wordsworth describes the quietude of the morning as being akin to a "garment" which lies over the city. This is obviously unusual and an unusual view of London for the poet, who feels a "calm" which is extremely "deep" in being able to witness such a sight. He notes that the air of the city, which is usually rendered dark with industry, is actually "smokeless" at this point in time, as if the factories, like the houses, are also asleep and thus unable to pump their usual smoke and smog into the atmosphere. Viewed from this vantage point, although London is one of the largest and busiest cities in the world at this point in time, the city appears to be what it is: a habitat that is part of a wider natural setting, giving way at its borders to the sky and to the fields around.