Looks to me like its been built in three stages:
1) Single storey garage with a flat roof
2) Garage Bricked up
3) Additional storey added
Which of course brings up a variety of issues:
Is the extension single skin?
Was the footing suitable for a single storey single skin garage or has it been increased to suit a 2 storey cavity wall construction and tiled pitched roof?
Obviously all of the above should have been completed completed by a competent builder who would have called in the local authority building control to check for compliance with building regs.
Definately get it thoroughly checked over.