By considering the urbanization process on an overland plane whereby a relatively rough permeable surface is replaced by a relatively smooth impermeable surface, the effect of urbanization on the time of concentration is assessed. In the assessment, two opposing urbanization sequences are considered: one from downstream to upstream, and the other from upstream to downstream. The assessment is carried out in terms of the individual effect as well as the combined effect of the Manning resistance coefficient and the runoff coefficient. For both urbanization sequences and for all degrees of urbanization, the assessment shows that surface conversion from rough to smooth or from permeable to impermeable causes the time of concentration to be shorter. Urbanization at the upstream end causes the greatest decrease in the time of concentration, and the reduction in surface roughness causes a greater decrease in time of concentration as compared to the reduction in permeability. Depending on the pre- and post-urbanization land surface conditions, the assessment shows that the time of concentration could decrease to 4–39% of its original value. Moreover, the time of concentration for a plane with urbanization from downstream to upstream could be more than four times longer than that for a plane with urbanization from upstream to downstream. Finally, there is a remarkable resemblance between the results of this study with those obtained from drainage basins in earlier studies, and it appears that the results from earlier studies are only applicable for the upstream to downstream urbanization sequence.