The MINIMUM level for damp proof courses is 150mm above adjoining ground level for building regulations. However, if their extension is butted up to your property without any vertical DPC between the buildings this would allow their rising damp to bridge your DPC, potentially intruducing damp to your dwelling.

If this is the case point it out to them immediately and if no result contact your local council's building control dept.

If they are building right up to the boundary line they should have served you with a Party Wall notice; if not served and agreed by you this is a potential lever as you could get the work stopped and object to the Notice, potentially costing them several £thousands of Party Wall Award costs and many weeks of delays!