We have investigated both experimentally and theoretically the formation and subsequent growth of surface relief structures on thin diblock copolymer films. The surface pattern appearing upon annealing is characterized by the average domain radius R. Using atomic force microscopy, we have obtained time-dependent topographs from which the critical exponent a in the growth law R proportional to t(a) has been determined. The values are significantly lower than the ones usually predicted for a in such films. We have discussed theoretically both the nucleation and formation stage of the process in detail, also considering the ideal situation (all domains belong to the same lamellar layer, and no additional defects are present). In particular the dependence of the growth law on the surface fraction Phi occupied by domains was addressed. For the ideal situation three main values of the exponent alpha were predicted : 1/4, 1, and 1/3. However, we show that several factors can be responsible for a decrease in the apparent exponent a, such as line defects, semi-islands, and the multilayer character of the domain pattern. A crucial role of the line defects, especially for small values of Phi, is predicted. Theoretical results are in qualitative agreement with the experimental data.