In the context of software cost estimation, system size is widely taken as a main driver of system development effort. But other structural design properties, such as coupling, cohesion, complexity have been suggested as additional cost factors. In this paper, we empirically investigate the relationship between class size and the development effort for a class, and what additional impact structural properties such as class coupling have on effort. We use Poisson regression and regression trees to build cost prediction models from size and design measures, and use these models to predict system development effort. We also investigate a recently suggested technique to combine regression trees with regression analysis, which aims at building more accurate models. Results indicate that fairly accurate predictions of class effort can be made based on simple measures of the class interface size alone (mean MREs below 30%). Effort predictions at the system level are even more accurate as, using Bootstrapping, the estimate 95% confidence interval for MREs is 3%-23%. But more sophisticated coupling and cohesion measures do not help to improve these predictions to a degree that would be practically significant. However, the use of hybrid models, combining Poisson regression and CART regression trees clearly improves the accuracy of the models, as compared to using Poisson regression alone.