soggy parts of lawn

My garden is on a slight decline so is good for drainage really.  Had a new lawn laid beginning of the year.  Have noticed that bottom of the garden is wet and boggy.  Have never noticed this before.  The bottom of the garden has a few small trees and bushes so doesnt get as much sun as rest of garden so I do not know if this could contribute to it, also the torrential rain we have been having.  How do I go about resolving this? I am having a shed built down there so I think builder can deal with where foundations will be going but I do not know what to do with the remaining bits of lawn