Firstly, you should check if a new development plan or draft development plan has come into force since you were originally granted permission. If so check if there are any changes which would affect the planning application (for instance, if the Planning Authority have since taken a strong stance against one-off housing in your area).
The new Environmental Protection Guidelines for single family dwellings have also come into force this year, and these may be applied retrospectively to your application, resulting in new percolation tests etc. being required.
Don’t be afraid to arrange a meeting with the relevant planning officer to discuss your problems. And have valid reasons for wishing to extend the permission period. Vagueness usually doesn’t play well in these situations.