Extensions vary from house to house. Depending on the layout of the house and the plot a rear, side, wraparound or front extension could be an option.
Double or Single storey, with single storey normally easier to get planning for an double storey a more economic build.
Permission is applied for using either a Householder Planning Application or Permitted Development.
Along with extensions, more often that not, an internal reconfiguration is undertaken to redesign the layout to suit the customers needs.