You will need to provide a damp proof course in the extension walls at least 150mm above ground level comprising of impervious sheet material, engineering brick or slates in suitable cement mortar or equivalent materials to prevent the passage of moisture.
Further guidance on suitable damp proof course materials can be found in British Standards Institution Published Document PD 6697: 2010 ‘Recommendations for the design of masonry structures to BS EN 1996-1-1 and BS EN 1996-2’ or in British Standards Institution BS 5628 Part 3: 2005 ‘Code of practice for the use of masonry – Materials and components, design and workmanship’. British Standards Institution publications are available from:
British Standards Institution
389 Chiswick High Road