To add an extension means to dig up and expand the basement; however, the cold joint between the old basement wall and the new might seep groundwater. How to avoid such a possibility?
In the case of the joint, or any seam, or discontinuity in a foundation wall it is advisable to join or extend the existing waterproofing system from the existing to the new foundation. There exists many different foundation coating, impermeabilisation membrane and weeping systems that can be aded to the outside of the foundation wall to "bridge" the gap and prevent water seepage from the point where the two foundations meet.
Treat the gap like a crack and waterproof the whole foundation for best results.
Hope this helps
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