Over on our sister blog, Photography & Architecture, we have an excellent post up informing Architects how to go about hiring a photographer. Julia Sabot interviewed Redeye Reps founder Maren Levinson about the process. You may find some good info in there or you may want to pass some advice onto potential clients:
There should be a cancellation policy or weather provision set up in advance, especially if there are multiple exterior shots on the list. Professional photographers are freelance. If they take your job, it is likely they are saying no to another. If you cancel without any notice due to weather or scheduling, they will want some sort of compensation for the day they did not accept another job. Mostly photographers will be reasonable about this and if they are local, could be ok with waiving it, but it should be discussed in advance.