If your building inspection has revealed a building issue that you weren’t previously aware of, you’ll need to thoroughly investigate the problem before deciding if it’s serious enough to invoke the ‘subject to...’ clause and decide to withdraw from the purchase.  You should speak to the building inspector and, if necessary, ask for a second, more detailed inspection to be carried out to find out the extent and cause of the rising damp and to determine if it can be easily rectified.  You may decide to proceed with the purchase regardless of whether the property you’re purchasing otherwise meets your standards.