I think we are missing a key aspect of what the OP is asking. This is closer to an SSRS question, I believe. There are 2 datasets in his report; 1 to provide the selected buildings as a drop-down list and another to provide the data based on the selection. Then, in SSRS you can allow "multiple selection" in the drop-down (report parameter). Check this out if you are trying to understand the multi-value parameter in SSRS:

http://social.msdn.microsoft.com/Forums/en-US/sqlreportingservices/thread/c02370b5-aeda-47ec-a3a8-43b2ec1e6c26/