Thank you very much for your advice.
As the building was originally one building and the garage has been built far less than 1/2 the width of this original dwelling. I understand that even though the building has subsequently been split into 2 separate houses the original building was built as one.
I think I will take some advice as we would prefer not to go for retrospective as its gives a bad name amongst local councillors and community and the other option is to remove or reduce it but this is costly and for the sake of 20cm doesn't seem worth it...!