Thank you guys.

It's actually 'quite' easy to do the houses. Draw the roads first, select them, expand the selection (for expample 6 px), choose a brush with this size or less (6 px or less) and choose the settings (jitters), turn selection into a path, fill path contour with brush. Finished. Now you have just to clean up the city. You get a well organized set up of quarters. For more randomness you still have to add and delete houses manually.